Apologies for my blank response...
I personally use a Linux HA + DRBD + LVM + Linux iSCSI solution,
and it works very well. Some things I took a lot of time to solve
include:
1) A whole bunch of network cabling/config issues, now using 10G
between DRBD, 10G iscsi server, and 2 x 1G for dom0's with
multipath.
2) Unexpected poor performance with HDD's, concurrent random
access from multiple domU's does not work well with HDD. My
solution was to upgrade to SSD.
3) Unexpected poor performance with SSD. This came down to testing
the wrong thing when calculating expected performance level. Test
with small (eg 4k) random read/write and use those results, unless
your VM's are only doing large read/write, and these really do get
merged, then you will find performance limited to the 4k request
size.
4) Still poor performance from SSD (DRBD). Change LVM so that it
is below DRBD. ie, one LV for each domU, then on top is DRBD for
each domU, then finally iscsi exports the DRBD devices.
5) Still poor performance from SSD (DRBD). DRBD needs to do it's
own write for every domU write, plus lvm does it's own, etc. Each
layer adds overhead. Solution for me was to disable DRBD
disk-barrier, disk-flushes, md-flushes
Other things that helped along the way include:
echo noop > /sys/block/${disk}/queue/scheduler
echo 128 > /sys/block/${disk}/queue/nr_requests
echo 4096 > /sys/block/md1/md/stripe_cache_size # RAID5, test
the correct value for your array
Currently, I have 8 dom0's with about 25 domU's and it is working
well, including transparently failing over on iscsi server
failure. If anyone wants more details, I'm happy to share.
Most of the above is perhaps not specific to xen, but storage in
general, but I hope it will be relevant here.
I'd also ask that if you get any direct response, that you please
summarise and send back to the list, and/or update the wiki so
others can more easily find the information.
Regards,
Adam
On 17/10/14 11:34, Eric wrote:
