Re: RAIDframe and alignment on 4K-sector disks?

["John D. Baker" <jdbaker%mylinuxisp.com@localhost>]

On Mon, 12 Jun 2017 04:55:52 -0000 (UTC), mlelstv%serpens.de@localhost (Michael
van Elst) wrote:

> Currently best practice for such a RAID is:
>  
> -> align the RAID partition(s) on each component to 4k

A given.  That's all these components are going to do, so 0 % 8 == 0.

> -> use a stripe size of 64k (maxphys), with RAID-5 on 3 disks that's
>    SecPerSU=64 (parity doesn't count).

Doesn't RAIDframe assume 512-byte sectors?  So, 64k stripe size would
be SecPerSU=128, yes?

> -> format the filesystem(s) with 64k blocks and 8k fragments (64k fragments
>    might be faster sometimes but waste disk space).

If one is making blocks=stripes, won't a single 8k fragment write provoke
an RMW operation at the RF level (operate on less than a full stripe)?
True, it won't cause RMW on the disk as it'll operate on full 4k sectors
across both data components.

I seem to recall another strategy to make frags=stripes so the minimal
FS operation would operate on a single stripe and a block would then be
multiple stripes.  Or does that incur too much parity overhead?

That was my idea in my initial query.  The idea of SecPerSU=16 being
the minimum that would have frags=stripes=8k on a 3-disk RAID-5.

Thanks for the feedback.

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