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btrfs-progs: mkfs: Warn user for minimal RAID5/6 devices setup

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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>
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
Qu Wenruo 2016-09-06 17:35:06 +02:00 committed by David Sterba
parent c2f3212502
commit 53c4e289c2
2 changed files with 20 additions and 6 deletions
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16
Documentation/mkfs.btrfs.asciidoc
View File

@ -263,18 +263,26 @@ 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 note 1)^
| 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 note 2)^
| RAID6 | 1 | 2 | 3 to N - 2 | 3/any ^(see note 3)^
|=============================================================
'Note:' DUP may exist on more than 1 device if it starts on a single device and
'Note 1:' 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.
'Note 2:' It's not recommended to use 2 devices with RAID5. In that case,
parity stripe will contain the same data as the data stripe, making RAID5
degraded to RAID1 with more overhead.
'Note 3:' It's also not recommended to use 3 devices with RAID6, unless you
want to get effectively 3 copies in a RAID1-like manner (but not exactly that).
N-copies RAID1 is not implemented.
DUP PROFILES ON A SINGLE DEVICE
-------------------------------

10
utils.c
View File

@ -3280,6 +3280,7 @@ 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:
@ -3294,8 +3295,7 @@ 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) {
@ -3315,6 +3315,12 @@ 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");