Adding to this thought, even if you are using a single replica for the cache pool, will ceph scrub the cached block against the base tier? What if you have corruption in your cache?
From: "Tyler Bishop" <tyler.bishop@xxxxxxxxxxxxxxxxx>
To: ceph-users@xxxxxxxxxxxxxx
Cc: "Sebastien han" <Sebastien.han@xxxxxxxxxxxx>
Sent: Sunday, January 17, 2016 3:47:13 PM
Subject: Ceph Cache pool redundancy requirements.
To: ceph-users@xxxxxxxxxxxxxx
Cc: "Sebastien han" <Sebastien.han@xxxxxxxxxxxx>
Sent: Sunday, January 17, 2016 3:47:13 PM
Subject: Ceph Cache pool redundancy requirements.
Based off Sebastiens design I had some thoughts: http://www.sebastien-han.fr/images/ceph-cache-pool-compute-design.png
Hypervisors are for obvious reason more susceptible to crashes and reboots for security updates. Since ceph is utilizing a standard pool for the cache tier it creates a requirement for placement group stability. IE: We cannot use a pool with only 1 PG replica required. The ideal configuration would be to utilize a single replica ssd cache pool as READ ONLY, and all writes will be sent to the base tier ssd journals, this way your getting quick acks and fast reads without any lost flash capacity for redundancy.
Has anyone tested a failure with a read only cache pool that utilizes a single replica? Does ceph simply fetch the data and place it to another pg? The cache pool should be able to sustain drive failures with 1 replica because its not needed for consistency.
Interesting topic here.. curious if anyone has tried this.
Our current architecture utilizes 48 hosts with 2x 1T SSD each as a 2 replica ssd pool. We have 4 host with 52x 6T disk for a capacity pool. We would like to run the base tier on the spindles with the SSD as a 100% utilized cache tier for busy pools.
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