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 <quwen...@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");
>
>
--
Steven Haigh
Email: net...@crc.id.au
Web: https://www.crc.id.au
Phone: (03) 9001 6090 - 0412 935 897
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