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SubjectRe: [ANNOUNCE] RAIF: Redundant Array of Independent Filesystems
> > Nikolai Joukov wrote:
> > > replication. In case of RAID4 and RAID5-like configurations, RAIF performed
> > > about two times *better* than software RAID and even better than an Adaptec
> > > 2120S RAID5 controller. This is because RAIF is located above file system
> > > caches and can cache parity as normal data when needed. We have more
> > > performance details in a technical report, if anyone is interested.
> >
> > This doesn't make sense to me. You do not want to cache the parity
> > data. It only needs to be used to validate the data blocks when the
> > stripe is read, and after that, you only want to cache the data, and
> > throw out the parity. Caching the parity as well will pollute the cache
> > and thus, should lower performance due to more important data being
> > thrown out.
> This happens automatically: unused parity pages are treated as unused
> pages and get reused to cache something else. Also, the parity
> never gets cached if you do not write the data (or recover the data).
> However, if you use the same parity page over and over you do not need to
> fetch it from the disk again.

To avoid confusion here: data recovery is not the only situation when it
is necessary to read the parity. Existing parity is also necessary for
writes that are smaller than the page size.

Nikolai Joukov, Ph.D.
Filesystems and Storage Laboratory
Stony Brook University
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