| Message ID | 20160902014145.7817-1-quwenruo@cn.fujitsu.com (mailing list archive) |
|---|---|
| State | Accepted |
| Headers | show |
Is it worthwhile adding a note that RAID5 / RAID6 may very well eat your data at this stage? On 02/09/16 11:41, Qu Wenruo wrote: > For RAID5, 2 devices setup is just RAID1 with more overhead. > For RAID6, 3 devices setup is RAID1 with 3 copies, not what most user > want. > > So warn user at mkfs time for such case, and add explain in man pages. > > Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com> > --- > Documentation/mkfs.btrfs.asciidoc | 15 +++++++++++---- > utils.c | 10 ++++++++-- > 2 files changed, 19 insertions(+), 6 deletions(-) > > diff --git a/Documentation/mkfs.btrfs.asciidoc b/Documentation/mkfs.btrfs.asciidoc > index 98fe694..846c08f 100644 > --- a/Documentation/mkfs.btrfs.asciidoc > +++ b/Documentation/mkfs.btrfs.asciidoc > @@ -263,18 +263,25 @@ There are the following block group types available: > .2+^.<h| Profile 3+^.^h| Redundancy .2+^.<h| Min/max devices > ^.^h| Copies ^.^h| Parity ^.<h| Striping > | single | 1 | | | 1/any > -| DUP | 2 / 1 device | | | 1/any ^(see note)^ > +| DUP | 2 / 1 device | | | 1/any ^(see note1)^ > | RAID0 | | | 1 to N | 2/any > | RAID1 | 2 | | | 2/any > | RAID10 | 2 | | 1 to N | 4/any > -| RAID5 | 1 | 1 | 2 to N - 1 | 2/any > -| RAID6 | 1 | 2 | 3 to N - 2 | 3/any > +| RAID5 | 1 | 1 | 2 to N - 1 | 2/any ^(see note2)^ > +| RAID6 | 1 | 2 | 3 to N - 2 | 3/any ^(see note3)^ > |============================================================= > > -'Note:' DUP may exist on more than 1 device if it starts on a single device and > +'Note1:' DUP may exist on more than 1 device if it starts on a single device and > another one is added. Since version 4.5.1, *mkfs.btrfs* will let you create DUP > on multiple devices. > > +'Note2:' It's not recommended to use 2 devices RAID5. In that case, > +parity stripe will contains the same data of data stripe, making RAID5 degraded > +to RAID1 with more overhead. > + > +'Note3:' It's also not recommended to use 3 devices RAID6, unless one wants to > +get 3 copies RAID1, which btrfs doesn't provide yet. > + > DUP PROFILES ON A SINGLE DEVICE > ------------------------------- > > diff --git a/utils.c b/utils.c > index 82f3376..1d6879a 100644 > --- a/utils.c > +++ b/utils.c > @@ -3314,6 +3314,7 @@ int test_num_disk_vs_raid(u64 metadata_profile, u64 data_profile, > u64 dev_cnt, int mixed, int ssd) > { > u64 allowed = 0; > + u64 profile = metadata_profile | data_profile; > > switch (dev_cnt) { > default: > @@ -3328,8 +3329,7 @@ int test_num_disk_vs_raid(u64 metadata_profile, u64 data_profile, > allowed |= BTRFS_BLOCK_GROUP_DUP; > } > > - if (dev_cnt > 1 && > - ((metadata_profile | data_profile) & BTRFS_BLOCK_GROUP_DUP)) { > + if (dev_cnt > 1 && profile & BTRFS_BLOCK_GROUP_DUP) { > warning("DUP is not recommended on filesystem with multiple devices"); > } > if (metadata_profile & ~allowed) { > @@ -3349,6 +3349,12 @@ int test_num_disk_vs_raid(u64 metadata_profile, u64 data_profile, > return 1; > } > > + if (dev_cnt == 3 && profile & BTRFS_BLOCK_GROUP_RAID6) { > + warning("RAID6 is not recommended on filesystem with 3 devices only"); > + } > + if (dev_cnt == 2 && profile & BTRFS_BLOCK_GROUP_RAID5) { > + warning("RAID5 is not recommended on filesystem with 2 devices only"); > + } > warning_on(!mixed && (data_profile & BTRFS_BLOCK_GROUP_DUP) && ssd, > "DUP may not actually lead to 2 copies on the device, see manual page"); > >
On Fri, Sep 02, 2016 at 11:59:07AM +1000, Steven Haigh wrote: > Is it worthwhile adding a note that RAID5 / RAID6 may very well eat your > data at this stage? This depends on the kernel implementation, so it needs to take that into account. There's a patchset in the works to automatically select mkfs features based on kernel so a warning for features like raid56 will be built on top of that. -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
diff --git a/Documentation/mkfs.btrfs.asciidoc b/Documentation/mkfs.btrfs.asciidoc index 98fe694..846c08f 100644 --- a/Documentation/mkfs.btrfs.asciidoc +++ b/Documentation/mkfs.btrfs.asciidoc @@ -263,18 +263,25 @@ There are the following block group types available: .2+^.<h| Profile 3+^.^h| Redundancy .2+^.<h| Min/max devices ^.^h| Copies ^.^h| Parity ^.<h| Striping | single | 1 | | | 1/any -| DUP | 2 / 1 device | | | 1/any ^(see note)^ +| DUP | 2 / 1 device | | | 1/any ^(see note1)^ | RAID0 | | | 1 to N | 2/any | RAID1 | 2 | | | 2/any | RAID10 | 2 | | 1 to N | 4/any -| RAID5 | 1 | 1 | 2 to N - 1 | 2/any -| RAID6 | 1 | 2 | 3 to N - 2 | 3/any +| RAID5 | 1 | 1 | 2 to N - 1 | 2/any ^(see note2)^ +| RAID6 | 1 | 2 | 3 to N - 2 | 3/any ^(see note3)^ |============================================================= -'Note:' DUP may exist on more than 1 device if it starts on a single device and +'Note1:' DUP may exist on more than 1 device if it starts on a single device and another one is added. Since version 4.5.1, *mkfs.btrfs* will let you create DUP on multiple devices. +'Note2:' It's not recommended to use 2 devices RAID5. In that case, +parity stripe will contains the same data of data stripe, making RAID5 degraded +to RAID1 with more overhead. + +'Note3:' It's also not recommended to use 3 devices RAID6, unless one wants to +get 3 copies RAID1, which btrfs doesn't provide yet. + DUP PROFILES ON A SINGLE DEVICE ------------------------------- diff --git a/utils.c b/utils.c index 82f3376..1d6879a 100644 --- a/utils.c +++ b/utils.c @@ -3314,6 +3314,7 @@ int test_num_disk_vs_raid(u64 metadata_profile, u64 data_profile, u64 dev_cnt, int mixed, int ssd) { u64 allowed = 0; + u64 profile = metadata_profile | data_profile; switch (dev_cnt) { default: @@ -3328,8 +3329,7 @@ int test_num_disk_vs_raid(u64 metadata_profile, u64 data_profile, allowed |= BTRFS_BLOCK_GROUP_DUP; } - if (dev_cnt > 1 && - ((metadata_profile | data_profile) & BTRFS_BLOCK_GROUP_DUP)) { + if (dev_cnt > 1 && profile & BTRFS_BLOCK_GROUP_DUP) { warning("DUP is not recommended on filesystem with multiple devices"); } if (metadata_profile & ~allowed) { @@ -3349,6 +3349,12 @@ int test_num_disk_vs_raid(u64 metadata_profile, u64 data_profile, return 1; } + if (dev_cnt == 3 && profile & BTRFS_BLOCK_GROUP_RAID6) { + warning("RAID6 is not recommended on filesystem with 3 devices only"); + } + if (dev_cnt == 2 && profile & BTRFS_BLOCK_GROUP_RAID5) { + warning("RAID5 is not recommended on filesystem with 2 devices only"); + } warning_on(!mixed && (data_profile & BTRFS_BLOCK_GROUP_DUP) && ssd, "DUP may not actually lead to 2 copies on the device, see manual page");
For RAID5, 2 devices setup is just RAID1 with more overhead. For RAID6, 3 devices setup is RAID1 with 3 copies, not what most user want. So warn user at mkfs time for such case, and add explain in man pages. Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com> --- Documentation/mkfs.btrfs.asciidoc | 15 +++++++++++---- utils.c | 10 ++++++++-- 2 files changed, 19 insertions(+), 6 deletions(-)