Significant writes to L2 during backup to NAS with WO strategy

[rutra80]

I'm testing PrimoCache with L1 & L2 Read & deferred Write cache. During backups I switch cache strategy to Write-data only to avoid filling of L2 cache with data being read from HDD and backed up onto NAS. Unfortunately there still are significant writes to L2 at about 1:10 ratio of data being read from HDD (so on 300GB of data read from HDD there are 30GB of writes to L2). There is nearly no data being written to HDD when this happens (confirmed by task manager and almost static Total Write Req/Done in PrimoCache), so what is being written to L2 and how to avoid it?

[rutra80]

My mistake, it is 1:1 ratio! It goes on and on after backup overlapping L2 several times with all the data backed up from HDD! Why does it happen with Write-only strategy??

[rutra80]

Strange, when I select Read-data only strategy nothing gets written to L2 during backup... Are "Read-data Only" & "Write-data Only" switched around or I don't understand something?

[InquiringMind]

"Write-only" means any data written to disk gets written to cache instead, while data read from disk is not cached. Such a policy makes sense for applications that keep overwriting files or that verify writes by reading the data again (in this case, PrimoCache boosts speed but may circumvent application data verification).

For backups, you should either

• pause caching completely while the backup is running;
• use backup software that works via a driver (Drive Snapshot is one example - Macrium Reflect Free *might* be another option if you prefer non-commercial software but I've not tried it myself). Using a driver for copying seems to bypass PrimoCache.

[Support]

@rutra80, this is a bug in PrimoCache L2 cache. L2 cache filling is not affected by cache strategy. So even you choose "Write-data only", PrimoCache still fetches read-data to L2 cache. We have noticed this bug and will fixed it. Thank you for the feedback!

As InquiringMind suggested, you may pause caching during backup.