[DRBD-user] iSCSI on DRBD replicated device : slow - DRBD barriers problem

[Valentin FERON]

Hello,

I am having performance problems with iSCSI and DRBD.

My setup is :

[client > write on xfs(no barrier) > LVM > iscsi initiator] > [iSCSI target
on /dev/drbd0 >  DRBD)]  :

When DRBD disk is configured in default mode
(with disk-flushes,md-flushes, disk-barrier), I have bad performance result.

Exemple :

>>
root at vferon:/mnt# dd if=/dev/zero of=test count=10k bs=64k
10240+0 enregistrements lus
10240+0 enregistrements écrits
671088640 octets (671 MB) copiés, 25,9988 s, 25,8 MB/s

>>
root at vferon:/mnt# dd if=/dev/zero of=test count=30k bs=64k
30720+0 enregistrements lus
30720+0 enregistrements écrits
2013265920 octets (2,0 GB) copiés, 129,431 s, 15,6 MB/s

---------------------------------------------------------------------------------------------------------
When DRBD disk is configured without barrier -
 disk {
no-disk-flushes;
no-md-flushes;
no-disk-barrier;
}

>>
root at vferon:/mnt# dd if=/dev/zero of=test count=10k bs=64k
10240+0 enregistrements lus
10240+0 enregistrements écrits
671088640 octets (671 MB) copiés, 11,8258 s, 56,7 MB/s
root at vferon:/mnt#

>>
root at vferon:/mnt# dd if=/dev/zero of=test count=30k bs=64k
30720+0 enregistrements lus
30720+0 enregistrements écrits
2013265920 octets (2,0 GB) copiés, 41,1322 s, 48,9 MB/s

------------------------------------------------------------------------------------------------------------

So i found a way to improve performance, but it is not safe for my data.

I know about the barrier mechanism and I would like to know why it act like
this with iSCSI and if any of you knows about the real cause of this if
there is one.

I tried an other setup using NFSv4 instead of iSCSI and I don't have a such
problem.

Thank you!

Feron Valentin
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