>>>>> "ps" == Peter Schuller <[EMAIL PROTECTED]> writes:
ps> The software raid in Linux does not support [write barriers]
ps> with raid5/raid6,
yeah i read this warning also and think it's a good argument for not
using it.
http://lwn.net/Articles/283161/
With RAID5 or RAID6 there is of course the write hole. But the way I
read it, just making soft partitions or mirrors with LVM2 breaks write
barriers too. From the comments:
-----8<-----
Q. is there any work going on towards making the barriers work on lvm
volumes?
A. Yes, but only single disk DM targets (e.g. linear), see:
http://lkml.org/lkml/2008/2/15/125
Unfortunately, this patch hasn't been pushed upstream and the DM
maintainer (agk) hasn't really commented on when it might.
-----8<-----
The downside is that if you do raidz2 above iscsi, I think iSCSI makes
one TCP circuit for each target, so the congestion avoidance will work
less well. maybe RED on the switch can help, and probably needs lots
[more than i have done] performance testing / comparison.
ps> iSCSI with respect to write barriers?
+1.
Does anyone even know of a good way to actually test it? So far it
seems the only way to know if your OS is breaking write barriers is to
trade gossip and guess.
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