dm-raid1 improvements

Posted Apr 19, 2008 7:31 UTC (Sat) by wolfgang.oertl (guest, #7418)
Parent article: Stable kernel 2.6.25 released

Another new feature worth mentioning is that drivers/md/dm-raid1.c (for mirroring) finally
does read balancing, not just reading from the first mirror (the older md implementation in
drivers/md/raid1.c has had this already).  The patch name is "dm raid1: handle read failures"
(2008-02-08).

dm-raid1 improvements

Posted Apr 19, 2008 16:15 UTC (Sat) by sbergman27 (guest, #10767) [Link] (5 responses)

What does this mean, exactly, for someone who set up a Fedora 8 server with software raid1
using the standard tools in anaconda?  Have I not been getting read balancing, but will get it
when F8 moves to 2.6.25?

dm-raid1 improvements

Posted Apr 20, 2008 8:10 UTC (Sun) by wolfgang.oertl (guest, #7418) [Link] (4 responses)

I'm not familiar with Fedora, but you can check whether your setup uses the device mapper or
the "md" drivers: type "mount" - if you see lines like /dev/mapper/some-thing then dm is used,
and you don't have read balancing yet. OTOH if /dev/md0 and similar is mounted, this is not an
issue.  Another option is to monitor drive activity with some GUI tool to see whether reads
are distributed to all drives of the raid, or have a look at /proc/partitions to check the
read counters.

dm-raid1 improvements

Posted Apr 20, 2008 10:46 UTC (Sun) by nix (subscriber, #2304) [Link] (3 responses)

Really?

nix@loki 10 /home/nix% df -Pk / Filesystem 1024-blocks Used Available Capacity Mounted on
/dev/raid/root 198344 47412 140692 26% /

Oh, I can't be using md then: odd, I didn't know they had RAID-5 md-raid yet.

Personalities : [raid1] [raid6] [raid5] [raid4]
md1 : active raid5 sda6[0] hdc5[3] sdb6[1]
76807296 blocks super 1.2 level 5, 64k chunk, algorithm 2 [3/3]
[UUU]
[...]

Hint: dm allows for a stack of block devices, and you can layer md both above it and (more usefully) below it. This particular system is using the fairly common configuration of LVM-atop-MD, thus is a dm-atop-md system.

Checking whether you are using the device mapper is not the same thing as checking whether you are using dm-raid.

dm-raid1 improvements

Posted Apr 20, 2008 17:59 UTC (Sun) by sbergman27 (guest, #10767) [Link] (2 responses)

Well, I am using the mapper.  And if I dd from the /dev/mapper/VolGroupxx/LogVolxx/ to
/dev/null and then do the same with /dev/sdax, which is the first member of the raid1 array, I
get about 61MB/sec in both cases.  So I guess I'm not getting read balancing in F8.  Or, come
to think of it, is the read balancing that fine-grained?  I recall somebody telling me, a
while back, that it only balanced reads from different processes, and not from the same
process, or something like that.

dm-raid1 improvements

Posted Apr 20, 2008 18:21 UTC (Sun) by nix (subscriber, #2304) [Link] (1 responses)

I can't imagine dm could tell which process was originating the request, 
even if it wanted to. It's all one by that point.

dm-raid1 improvements

Posted Apr 20, 2008 18:44 UTC (Sun) by sbergman27 (guest, #10767) [Link]

Seemed odd to me, too.  Another odd thing is that this kernel release, which I had not really
thought was anything all that special, actually has not one, but at least two bombshell
features.  How did I miss the loud outcry which surely occurred when read balance support in
raid1 was lost?  And why have both its return, and the new TASK_KILLABLE functionality gotten
so little notice?