Hi list,
I'm not sure this is the correct list to post my question; if so, feel
free to ignore this message.
On XFS FAQs I (rightfully) read that barrier should be *always* enabled
(and they are, by default), unless a write-loss protected writeback
cache is used at the RAID card level. So far, so good.
Newer HDDs has themselves have a non-volatile cache (NVC) of some sort,
basically:
- DRAM + eMCL: in a powerloss event, the DRAM cache is immediately
dumped on the eMLC flash;
- DRAM + NOR flash: NOR mirrors a (small) portion of the DRAM cache,
used for write acceleration purpose;
- dedicated "dump" areas on the disks: they effectively mirror a portion
of the DRAM cache.
My question is: do you think it is safe to disable barriers, both for
XFS and in general terms, on these disks? Or they should be considered
as the same "dumb" unprotected DRAM caches found on classical HDD?
From a side, these *are* powerloss-protected caches. Problem is that all
these power-protection schemes are considered "secret sauce / trade
secret" by HDD vendors and, for this reason, there are very little (if
any) informations on their inner working.
Regards.
--
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti@xxxxxxxxxx - info@xxxxxxxxxx
GPG public key ID: FF5F32A8
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