| Message ID | 08ddc67b-392f-efe0-ffd2-a7295a42bac6@redhat.com (mailing list archive) |
|---|---|
| State | Deferred, archived |
| Headers | show |
| Series | [RFC] mkfs.xfs: do not set log stripe unit for probed sw <= 2 | expand |
On 5/27/20 12:26 PM, Eric Sandeen wrote: > If the stripe width of a device is only 2x or 1x the stripe unit, there > is no parity disk on this device, and setting a larger log stripe unit > will not avoid any RMW cycles. However, a large log stripe unit does > have significant penalties for IO amplification because every log write > will be rounded up to the log stripe unit. > > This was recently highlighted by a user running bonnie++ in sync mode, > where the default RAID10 geometry of 256k/512k yielded results which > were 4x slower than a smaller log stripe unit. While bonnie++ may not > be the benchmark of choice, it does highlight this issue. > > Because a larger log stripe unit yields no RMW benefit on a device with > no parity disks, avoid setting in these cases. NAK I wasn't thinking about stacked parity raids like RAID50 (thanks Dave). So we cannot infer anything about parity disks from the top-level geometry, and so the proposed optimization is not valid. -Eric
diff --git a/mkfs/xfs_mkfs.c b/mkfs/xfs_mkfs.c index 2e6cd280..4da69b29 100644 --- a/mkfs/xfs_mkfs.c +++ b/mkfs/xfs_mkfs.c @@ -2407,13 +2407,15 @@ _("log stripe unit (%d) must be a multiple of the block size (%d)\n"), } /* - * check that log sunit is modulo fsblksize or default it to dsunit. + * check that log sunit is modulo fsblksize or default it to dsunit + * if this looks like a parity device (swidth > 2x sunit). */ if (lsunit) { /* convert from 512 byte blocks to fs blocks */ cfg->lsunit = DTOBT(lsunit, cfg->blocklog); } else if (cfg->sb_feat.log_version == 2 && - cfg->loginternal && cfg->dsunit) { + cfg->loginternal && cfg->dsunit && + (cfg->dswidth / cfg->dsunit > 2)) { /* lsunit and dsunit now in fs blocks */ cfg->lsunit = cfg->dsunit; }
If the stripe width of a device is only 2x or 1x the stripe unit, there is no parity disk on this device, and setting a larger log stripe unit will not avoid any RMW cycles. However, a large log stripe unit does have significant penalties for IO amplification because every log write will be rounded up to the log stripe unit. This was recently highlighted by a user running bonnie++ in sync mode, where the default RAID10 geometry of 256k/512k yielded results which were 4x slower than a smaller log stripe unit. While bonnie++ may not be the benchmark of choice, it does highlight this issue. Because a larger log stripe unit yields no RMW benefit on a device with no parity disks, avoid setting in these cases. Signed-off-by: Eric Sandeen <sandeen@redhat.com> --- ... thoughts? Am I missing a reason why we /would/ still want lsunit in this case?