On Sun, 30 Nov 2014 12:11:47 +0100
Gour <gour@xxxxxxxxxxxx> wrote:
> However, I wonder if there are some 'cons' in having raid-1 partition
> under mdadm and not using native mirroring capabilities of btrfs fs?
Pros:
* mdadm RAID has much better read balancing;
Btrfs reads are satisfied from what's in effect a random drive (PID-based
balancing of threads to drives), mdadm reads from the less-loaded drive.
Also mdadm has a way to specify some RAID1 array members as to be never
used for reads if at all possible ("write-mostly"), which helps in RAID1 of
HDD and SSD.
* mdadm RAID has much better write submission;
In my experience [1] Btrfs RAID1 on heavy write operations first writes to
one drive, then to another. The whole process takes up to 2x longer than
with a single drive. On the other hand mdadm writes to both drives
simultaneously.
[1] https://www.mail-archive.com/linux-btrfs@xxxxxxxxxxxxxxx/msg34103.html
Con:
* You only get the ability to recover from a checksum failure with Btrfs
RAID1, not with mdadm RAID1 (see Russell's reply).
--
With respect,
Roman
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