Hello,
We are building a new Ceph cluster and have a few questions regarding
the use of write barriers, controler cache, and disk cache (buffer).
Greg said that barriers should be used
(http://lists.ceph.com/pipermail/ceph-users-ceph.com/2013-July/002854.html)
for data safety which is the default mount option with XFS.
But what if we're using RAID controlers with battery backed up cache ?
XFS faq recommends to disable barriers when using battery backuped up
RAID controlers AND to make sure to disable disks's cache. Which we are
able to do on our PERC controlers (Dell R730xd).
(http://xfs.org/index.php/XFS_FAQ#Write_barrier_support). Also we should
make sure to disable battery relearning cycles.
Questions are :
Is it safe to use the controler cache in front of SAS/SATA data disks ?
(Our tests showed 1.95x more read/write IOPS when using cache)
Is it safe to use the controler cache in front of SSD metadata disks ?
(Our tests showed 1.38x more read/write IOPS when using cache). SSD
metadata disks are protected from power loss
(http://toshiba.semicon-storage.com/us/product/storage-products/enterprise-ssd/px02smb-px02smfxxx.html)
When using the controler cache (with multiple single drive RAID0
volumes), should we disable disk cache in any scenario ? Should we use
barriers or not ?
It's not clear to me whether the barrier mechanism will apply to the
controler cache or through the controler cache up to the physical disks.
Regards,
Frederic.
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