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@xxxxxxxxxxxxxx>
---
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");
--
2.9.3
--
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