From patchwork Mon Jun 1 06:43:53 2015 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: "Seymour, Shane M" X-Patchwork-Id: 6517721 Return-Path: X-Original-To: patchwork-linux-scsi@patchwork.kernel.org Delivered-To: patchwork-parsemail@patchwork1.web.kernel.org Received: from mail.kernel.org (mail.kernel.org [198.145.29.136]) by patchwork1.web.kernel.org (Postfix) with ESMTP id 166D39F1CC for ; Mon, 1 Jun 2015 06:44:39 +0000 (UTC) Received: from mail.kernel.org (localhost [127.0.0.1]) by mail.kernel.org (Postfix) with ESMTP id 4FB89205EE for ; Mon, 1 Jun 2015 06:44:37 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 03F5020582 for ; Mon, 1 Jun 2015 06:44:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754004AbbFAGoe (ORCPT ); Mon, 1 Jun 2015 02:44:34 -0400 Received: from g4t3425.houston.hp.com ([15.201.208.53]:31450 "EHLO g4t3425.houston.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751891AbbFAGoc convert rfc822-to-8bit (ORCPT ); Mon, 1 Jun 2015 02:44:32 -0400 Received: from G4W6310.americas.hpqcorp.net (g4w6310.houston.hp.com [16.210.26.217]) (using TLSv1 with cipher AES128-SHA (128/128 bits)) (No client certificate requested) by g4t3425.houston.hp.com (Postfix) with ESMTPS id CD593CA; Mon, 1 Jun 2015 06:44:31 +0000 (UTC) Received: from G4W6305.americas.hpqcorp.net (16.210.26.230) by G4W6310.americas.hpqcorp.net (16.210.26.217) with Microsoft SMTP Server (TLS) id 14.3.169.1; Mon, 1 Jun 2015 06:43:54 +0000 Received: from G9W0748.americas.hpqcorp.net ([169.254.10.200]) by G4W6305.americas.hpqcorp.net ([16.210.26.230]) with mapi id 14.03.0169.001; Mon, 1 Jun 2015 06:43:53 +0000 From: "Seymour, Shane M" To: "linux-scsi@vger.kernel.org" CC: Christoph Hellwig , "Laurence Oberman (loberman@redhat.com)" , "Kai.Makisara@kolumbus.fi" , "James E.J. Bottomley (JBottomley@parallels.com)" Subject: [PATCH v9] st: implement tape statistics Thread-Topic: [PATCH v9] st: implement tape statistics Thread-Index: AdCcNcUtXCu6l03oR+uVop2y4fhv/w== Date: Mon, 1 Jun 2015 06:43:53 +0000 Message-ID: Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-originating-ip: [16.210.48.19] MIME-Version: 1.0 Sender: linux-scsi-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-scsi@vger.kernel.org X-Spam-Status: No, score=-6.9 required=5.0 tests=BAYES_00, RCVD_IN_DNSWL_HI, T_RP_MATCHES_RCVD, UNPARSEABLE_RELAY autolearn=unavailable version=3.3.1 X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on mail.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP st: implement tape statistics This patch implements tape statistics in the st module via sysfs. Current no statistics are available for tape I/O and there is no easy way to reuse the block layer statistics for tape as tape is a character device and does not have perform I/O in sector sized chunks (the size of the data written to tape can change). For tapes we also need extra stats related to things like tape movement (via other I/O). There have been multiple end users requesting statistics including AT&T (and some HP customers who have not given permission to be named). It is impossible for them to investigate any issues related to tape performance in a non-invasive way. Signed-off-by: Shane Seymour Tested-by: Shane Seymour Reviewed-by: Christoph Hellwig --- Changes from v8 - Fixed warnings when printing statistics information in 32bit mode by providing a PRId64 definition for 64 and 32 kernels. Used PRId64 so if someone introduces those defines they will be able to find this code and remove this definition of it. - Updated kernel version given in sysfs ABI doco from ?? to 4.2. Changes from v7 - Changed name of patch added colon - Added better description of patch - Removed tested by (myself) line Changes from v6 - Removed tested by Laurence Oberman since the code has changed significantly. - Changed code to use ktime so time resolution is now in ns (Robert Elliot) for virtual tape drives that may be able to submit and complete an I/O within a jiffy. - Changed code to use atomic64_t types for statistics to ensure consistency between being updated and being read via sysfs (Robert Elliot). This should help ensure that we do not have MP or multi-threaded issues when accessing the data. - Some tape stats structure members changed names after they changed types to make their usage clearer. - Sysfs stats files that changed from ms to ns changed names to indicate that they were in ns now. - Fixed typo in incremnented in sysfs description (Robert Elliot) - Shorten statistics directory name to stats (Robert Elliot) - Moved the update of in_flight so it's incremenented before we update any statstics and decrememented after we're done. - Missing braces between if/else added to more clearly show where the if/else is. - Tested on 4.0.0-next-20150423+ -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html --- a/drivers/scsi/st.c 2015-04-22 18:13:13.097855752 -0500 +++ b/drivers/scsi/st.c 2015-05-31 17:52:08.891204751 -0500 @@ -471,6 +471,47 @@ static void st_release_request(struct st kfree(streq); } +static void st_do_stats(struct scsi_tape *STp, struct request *req) +{ + ktime_t now; + + now = ktime_get(); + if (req->cmd[0] == WRITE_6) { + now = ktime_sub(now, STp->stats->write_time); + atomic64_add(ktime_to_ns(now), &STp->stats->tot_write_time); + atomic64_add(ktime_to_ns(now), &STp->stats->tot_io_time); + atomic64_inc(&STp->stats->write_cnt); + if (req->errors) { + atomic64_add(atomic_read(&STp->stats->last_write_size) + - STp->buffer->cmdstat.residual, + &STp->stats->write_byte_cnt); + if (STp->buffer->cmdstat.residual > 0) + atomic64_inc(&STp->stats->resid_cnt); + } else + atomic64_add(atomic_read(&STp->stats->last_write_size), + &STp->stats->write_byte_cnt); + } else if (req->cmd[0] == READ_6) { + now = ktime_sub(now, STp->stats->read_time); + atomic64_add(ktime_to_ns(now), &STp->stats->tot_read_time); + atomic64_add(ktime_to_ns(now), &STp->stats->tot_io_time); + atomic64_inc(&STp->stats->read_cnt); + if (req->errors) { + atomic64_add(atomic_read(&STp->stats->last_read_size) + - STp->buffer->cmdstat.residual, + &STp->stats->read_byte_cnt); + if (STp->buffer->cmdstat.residual > 0) + atomic64_inc(&STp->stats->resid_cnt); + } else + atomic64_add(atomic_read(&STp->stats->last_read_size), + &STp->stats->read_byte_cnt); + } else { + now = ktime_sub(now, STp->stats->other_time); + atomic64_add(ktime_to_ns(now), &STp->stats->tot_io_time); + atomic64_inc(&STp->stats->other_cnt); + } + atomic64_dec(&STp->stats->in_flight); +} + static void st_scsi_execute_end(struct request *req, int uptodate) { struct st_request *SRpnt = req->end_io_data; @@ -480,6 +521,8 @@ static void st_scsi_execute_end(struct r STp->buffer->cmdstat.midlevel_result = SRpnt->result = req->errors; STp->buffer->cmdstat.residual = req->resid_len; + st_do_stats(STp, req); + tmp = SRpnt->bio; if (SRpnt->waiting) complete(SRpnt->waiting); @@ -496,6 +539,7 @@ static int st_scsi_execute(struct st_req struct rq_map_data *mdata = &SRpnt->stp->buffer->map_data; int err = 0; int write = (data_direction == DMA_TO_DEVICE); + struct scsi_tape *STp = SRpnt->stp; req = blk_get_request(SRpnt->stp->device->request_queue, write, GFP_KERNEL); @@ -516,6 +560,17 @@ static int st_scsi_execute(struct st_req } } + atomic64_inc(&STp->stats->in_flight); + if (cmd[0] == WRITE_6) { + atomic_set(&STp->stats->last_write_size, bufflen); + STp->stats->write_time = ktime_get(); + } else if (cmd[0] == READ_6) { + atomic_set(&STp->stats->last_read_size, bufflen); + STp->stats->read_time = ktime_get(); + } else { + STp->stats->other_time = ktime_get(); + } + SRpnt->bio = req->bio; req->cmd_len = COMMAND_SIZE(cmd[0]); memset(req->cmd, 0, BLK_MAX_CDB); @@ -4222,6 +4277,12 @@ static int st_probe(struct device *dev) } tpnt->index = error; sprintf(disk->disk_name, "st%d", tpnt->index); + tpnt->stats = kzalloc(sizeof(struct scsi_tape_stats), GFP_KERNEL); + if (tpnt->stats == NULL) { + sdev_printk(KERN_ERR, SDp, + "st: Can't allocate statistics.\n"); + goto out_idr_remove; + } dev_set_drvdata(dev, tpnt); @@ -4241,6 +4302,8 @@ static int st_probe(struct device *dev) out_remove_devs: remove_cdevs(tpnt); + kfree(tpnt->stats); +out_idr_remove: spin_lock(&st_index_lock); idr_remove(&st_index_idr, tpnt->index); spin_unlock(&st_index_lock); @@ -4298,6 +4361,7 @@ static void scsi_tape_release(struct kre disk->private_data = NULL; put_disk(disk); + kfree(tpnt->stats); kfree(tpnt); return; } @@ -4513,6 +4577,190 @@ options_show(struct device *dev, struct } static DEVICE_ATTR_RO(options); +/* Support for tape stats */ + +#if (BITS_PER_LONG == 64) +#define PRId64 "ld" +#else +#define PRId64 "lld" +#endif + +/** + * read_cnt_show - return read count - count of reads made from tape drive + * @dev: struct device + * @attr: attribute structure + * @buf: buffer to return formatted data in + */ +static ssize_t read_cnt_show(struct device *dev, + struct device_attribute *attr, char *buf) +{ + struct st_modedef *STm = dev_get_drvdata(dev); + + return sprintf(buf, "%"PRId64, + atomic64_read(&STm->tape->stats->read_cnt)); +} +static DEVICE_ATTR_RO(read_cnt); + +/** + * read_byte_cnt_show - return read byte count - tape drives + * may use blocks less than 512 bytes this gives the raw byte count of + * of data read from the tape drive. + * @dev: struct device + * @attr: attribute structure + * @buf: buffer to return formatted data in + */ +static ssize_t read_byte_cnt_show(struct device *dev, + struct device_attribute *attr, char *buf) +{ + struct st_modedef *STm = dev_get_drvdata(dev); + + return sprintf(buf, "%"PRId64, + atomic64_read(&STm->tape->stats->read_byte_cnt)); +} +static DEVICE_ATTR_RO(read_byte_cnt); + +/** + * read_us_show - return read us - overall time spent waiting on reads in ns. + * @dev: struct device + * @attr: attribute structure + * @buf: buffer to return formatted data in + */ +static ssize_t read_ns_show(struct device *dev, + struct device_attribute *attr, char *buf) +{ + struct st_modedef *STm = dev_get_drvdata(dev); + + return sprintf(buf, "%"PRId64, + atomic64_read(&STm->tape->stats->tot_read_time)); +} +static DEVICE_ATTR_RO(read_ns); + +/** + * write_cnt_show - write count - number of user calls + * to write(2) that have written data to tape. + * @dev: struct device + * @attr: attribute structure + * @buf: buffer to return formatted data in + */ +static ssize_t write_cnt_show(struct device *dev, + struct device_attribute *attr, char *buf) +{ + struct st_modedef *STm = dev_get_drvdata(dev); + + return sprintf(buf, "%"PRId64, + atomic64_read(&STm->tape->stats->write_cnt)); +} +static DEVICE_ATTR_RO(write_cnt); + +/** + * write_byte_cnt_show - write byte count - raw count of + * bytes written to tape. + * @dev: struct device + * @attr: attribute structure + * @buf: buffer to return formatted data in + */ +static ssize_t write_byte_cnt_show(struct device *dev, + struct device_attribute *attr, char *buf) +{ + struct st_modedef *STm = dev_get_drvdata(dev); + + return sprintf(buf, "%"PRId64, + atomic64_read(&STm->tape->stats->write_byte_cnt)); +} +static DEVICE_ATTR_RO(write_byte_cnt); + +/** + * write_ns_show - write ns - number of nanoseconds waiting on write + * requests to complete. + * @dev: struct device + * @attr: attribute structure + * @buf: buffer to return formatted data in + */ +static ssize_t write_ns_show(struct device *dev, + struct device_attribute *attr, char *buf) +{ + struct st_modedef *STm = dev_get_drvdata(dev); + + return sprintf(buf, "%"PRId64, + atomic64_read(&STm->tape->stats->tot_write_time)); +} +static DEVICE_ATTR_RO(write_ns); + +/** + * in_flight_show - number of I/Os currently in flight - + * in most cases this will be either 0 or 1. It may be higher if someone + * has also issued other SCSI commands such as via an ioctl. + * @dev: struct device + * @attr: attribute structure + * @buf: buffer to return formatted data in + */ +static ssize_t in_flight_show(struct device *dev, + struct device_attribute *attr, char *buf) +{ + struct st_modedef *STm = dev_get_drvdata(dev); + + return sprintf(buf, "%"PRId64, + atomic64_read(&STm->tape->stats->in_flight)); +} +static DEVICE_ATTR_RO(in_flight); + +/** + * io_ns_show - io wait ns - this is the number of ns spent + * waiting on all I/O to complete. This includes tape movement commands + * such as rewinding, seeking to end of file or tape, it also includes + * read and write. To determine the time spent on tape movement + * subtract the read and write ns from this value. + * @dev: struct device + * @attr: attribute structure + * @buf: buffer to return formatted data in + */ +static ssize_t io_ns_show(struct device *dev, + struct device_attribute *attr, char *buf) +{ + struct st_modedef *STm = dev_get_drvdata(dev); + + return sprintf(buf, "%"PRId64, + atomic64_read(&STm->tape->stats->tot_io_time)); +} +static DEVICE_ATTR_RO(io_ns); + +/** + * other_cnt_show - other io count - this is the number of + * I/O requests other than read and write requests. + * Typically these are tape movement requests but will include driver + * tape movement. This includes only requests issued by the st driver. + * @dev: struct device + * @attr: attribute structure + * @buf: buffer to return formatted data in + */ +static ssize_t other_cnt_show(struct device *dev, + struct device_attribute *attr, char *buf) +{ + struct st_modedef *STm = dev_get_drvdata(dev); + + return sprintf(buf, "%"PRId64, + atomic64_read(&STm->tape->stats->other_cnt)); +} +static DEVICE_ATTR_RO(other_cnt); + +/** + * resid_cnt_show - A count of the number of times we get a residual + * count - this should indicate someone issuing reads larger than the + * block size on tape. + * @dev: struct device + * @attr: attribute structure + * @buf: buffer to return formatted data in + */ +static ssize_t resid_cnt_show(struct device *dev, + struct device_attribute *attr, char *buf) +{ + struct st_modedef *STm = dev_get_drvdata(dev); + + return sprintf(buf, "%"PRId64, + atomic64_read(&STm->tape->stats->resid_cnt)); +} +static DEVICE_ATTR_RO(resid_cnt); + static struct attribute *st_dev_attrs[] = { &dev_attr_defined.attr, &dev_attr_default_blksize.attr, @@ -4521,7 +4769,35 @@ static struct attribute *st_dev_attrs[] &dev_attr_options.attr, NULL, }; -ATTRIBUTE_GROUPS(st_dev); + +static struct attribute *st_stats_attrs[] = { + &dev_attr_read_cnt.attr, + &dev_attr_read_byte_cnt.attr, + &dev_attr_read_ns.attr, + &dev_attr_write_cnt.attr, + &dev_attr_write_byte_cnt.attr, + &dev_attr_write_ns.attr, + &dev_attr_in_flight.attr, + &dev_attr_io_ns.attr, + &dev_attr_other_cnt.attr, + &dev_attr_resid_cnt.attr, + NULL, +}; + +static struct attribute_group stats_group = { + .name = "stats", + .attrs = st_stats_attrs, +}; + +static struct attribute_group st_group = { + .attrs = st_dev_attrs, +}; + +static const struct attribute_group *st_dev_groups[] = { + &st_group, + &stats_group, + NULL, +}; /* The following functions may be useful for a larger audience. */ static int sgl_map_user_pages(struct st_buffer *STbp, --- a/drivers/scsi/st.h 2015-04-22 18:13:13.097855752 -0500 +++ b/drivers/scsi/st.h 2015-04-22 18:15:58.238844505 -0500 @@ -92,6 +92,27 @@ struct st_partstat { int drv_file; }; +/* Tape statistics */ +struct scsi_tape_stats { + atomic64_t read_byte_cnt; /* bytes read */ + atomic64_t write_byte_cnt; /* bytes written */ + atomic64_t in_flight; /* Number of I/Os in flight */ + atomic64_t read_cnt; /* Count of read requests */ + atomic64_t write_cnt; /* Count of write requests */ + atomic64_t other_cnt; /* Count of other requests either + * implicit or from user space + * ioctl. */ + atomic64_t resid_cnt; /* Count of resid_len > 0 */ + atomic64_t tot_read_time; /* ktime spent completing reads */ + atomic64_t tot_write_time; /* ktime spent completing writes */ + atomic64_t tot_io_time; /* ktime spent doing any I/O */ + ktime_t read_time; /* holds ktime request was queued */ + ktime_t write_time; /* holds ktime request was queued */ + ktime_t other_time; /* holds ktime request was queued */ + atomic_t last_read_size; /* Number of bytes issued for last read */ + atomic_t last_write_size; /* Number of bytes issued for last write */ +}; + #define ST_NBR_PARTITIONS 4 /* The tape drive descriptor */ @@ -171,6 +192,7 @@ struct scsi_tape { #endif struct gendisk *disk; struct kref kref; + struct scsi_tape_stats *stats; }; /* Bit masks for use_pf */ --- a/Documentation/scsi/st.txt 2015-04-22 18:13:09.496855998 -0500 +++ b/Documentation/scsi/st.txt 2015-04-22 18:15:58.239844505 -0500 @@ -151,6 +151,65 @@ A link named 'tape' is made from the SCS directory corresponding to the mode 0 auto-rewind device (e.g., st0). +SYSFS AND STATISTICS FOR TAPE DEVICES + +The st driver maintains statistics for tape drives inside the sysfs filesystem. +The following method can be used to locate the statistics that are +available (assuming that sysfs is mounted at /sys): + +1. Use opendir(3) on the directory /sys/class/scsi_tape +2. Use readdir(3) to read the directory contents +3. Use regcomp(3)/regexec(3) to match directory entries to the extended + regular expression "^st[0-9]+$" +4. Access the statistics from the /sys/class/scsi_tape//stats + directory (where is a directory entry from /sys/class/scsi_tape + that matched the extended regular expression) + +The reason for using this approach is that all the character devices +pointing to the same tape drive use the same statistics. That means +that st0 would have the same statistics as nst0. + +The directory contains the following statistics files: + +1. in_flight - The number of I/Os currently outstanding to this device. +2. io_ns - The amount of time spent waiting (in nanoseconds) for all I/O + to complete (including read and write). This includes tape movement + commands such as seeking between file or set marks and implicit tape + movement such as when rewind on close tape devices are used. +3. other_cnt - The number of I/Os issued to the tape drive other than read or + write commands. The time taken to complete these commands uses the + following calculation io_ms-read_ms-write_ms. +4. read_byte_cnt - The number of bytes read from the tape drive. +5. read_cnt - The number of read requests issued to the tape drive. +6. read_ns - The amount of time (in nanoseconds) spent waiting for read + requests to complete. +7. write_byte_cnt - The number of bytes written to the tape drive. +8. write_cnt - The number of write requests issued to the tape drive. +9. write_ns - The amount of time (in nanoseconds) spent waiting for write + requests to complete. +10. resid_cnt - The number of times during a read or write we found + the residual amount to be non-zero. This should mean that a program + is issuing a read larger thean the block size on tape. For write + not all data made it to tape. + +Note: The in_flight value is incremented when an I/O starts the I/O +itself is not added to the statistics until it completes. + +The total of read_cnt, write_cnt, and other_cnt may not total to the same +value as iodone_cnt at the device level. The tape statistics only count +I/O issued via the st module. + +When read the statistics may not be temporally consistent while I/O is in +progress. The individual values are read and written to atomically however +when reading them back via sysfs they may be in the process of being +updated when starting an I/O or when it is completed. + +The value shown in in_flight is incremented before any statstics are +updated and decremented when an I/O completes after updating statistics. +The value of in_flight is 0 when there are no I/Os outstanding that are +issued by the st driver. Tape statistics do not take into account any +I/O performed via the sg device. + BSD AND SYS V SEMANTICS The user can choose between these two behaviours of the tape driver by --- a/Documentation/ABI/testing/sysfs-class-scsi_tape 1969-12-31 18:00:00.000000000 -0600 +++ b/Documentation/ABI/testing/sysfs-class-scsi_tape 2015-05-31 17:55:10.045192413 -0500 @@ -0,0 +1,109 @@ +What: /sys/class/scsi_tape/*/stats/in_flight +Date: Apr 2015 +KernelVersion: 4.2 +Contact: Shane Seymour +Description: + Show the number of I/Os currently in-flight between the st + module and the SCSI mid-layer. +Users: + + +What: /sys/class/scsi_tape/*/stats/io_ns +Date: Apr 2015 +KernelVersion: 4.2 +Contact: Shane Seymour +Description: + Shows the total amount of time spent waiting for all I/O + to and from the tape drive to complete. This includes all + reads, writes, and other SCSI commands issued to the tape + drive. An example of other SCSI commands would be tape + movement such as a rewind when a rewind tape device is + closed. This item is measured in nanoseconds. + + To determine the amount of time spent waiting for other I/O + to complete subtract read_ns and write_ns from this value. +Users: + + +What: /sys/class/scsi_tape/*/stats/other_cnt +Date: Apr 2015 +KernelVersion: 4.2 +Contact: Shane Seymour +Description: + The number of I/O requests issued to the tape drive other + than SCSI read/write requests. +Users: + + +What: /sys/class/scsi_tape/*/stats/read_byte_cnt +Date: Apr 2015 +KernelVersion: 4.2 +Contact: Shane Seymour +Description: + Shows the total number of bytes requested from the tape drive. + This value is presented in bytes because tape drives support + variable length block sizes. +Users: + + +What: /sys/class/scsi_tape/*/stats/read_cnt +Date: Apr 2015 +KernelVersion: 4.2 +Contact: Shane Seymour +Description: + Shows the total number of read requests issued to the tape + drive. +Users: + + +What: /sys/class/scsi_tape/*/stats/read_ns +Date: Apr 2015 +KernelVersion: 4.2 +Contact: Shane Seymour +Description: + Shows the total amount of time in nanoseconds waiting for + read I/O requests to complete. +Users: + + +What: /sys/class/scsi_tape/*/stats/write_byte_cnt +Date: Apr 2015 +KernelVersion: 4.2 +Contact: Shane Seymour +Description: + Shows the total number of bytes written to the tape drive. + This value is presented in bytes because tape drives support + variable length block sizes. +Users: + + +What: /sys/class/scsi_tape/*/stats/write_cnt +Date: Apr 2015 +KernelVersion: 4.2 +Contact: Shane Seymour +Description: + Shows the total number of write requests issued to the tape + drive. +Users: + + +What: /sys/class/scsi_tape/*/stats/write_ms +Date: Apr 2015 +KernelVersion: 4.2 +Contact: Shane Seymour +Description: + Shows the total amount of time in nanoseconds waiting for + write I/O requests to complete. +Users: + + +What: /sys/class/scsi_tape/*/stats/resid_cnt +Date: Apr 2015 +KernelVersion: 4.2 +Contact: Shane Seymour +Description: + Shows the number of times we found that a residual >0 + was found when the SCSI midlayer indicated that there was + an error. For reads this may be a case of someone issuing + reads greater than the block size. +Users: