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Re: RAID6 r-m-w, op-journaled fs, SSDs

To: Linux RAID <linux-raid@xxxxxxxxxxxxxxx>, Linux fs JFS <jfs-discussion@xxxxxxxxxxxxxxxxxxxxx>, Linux fs XFS <xfs@xxxxxxxxxxx>
Subject: Re: RAID6 r-m-w, op-journaled fs, SSDs
From: pg_xf2@xxxxxxxxxxxxxxxxxx (Peter Grandi)
Date: Sun, 1 May 2011 16:31:34 +0100
In-reply-to: <20110501082717.5116e575@xxxxxxxxxxxxxx>
References: <19900.10868.583555.849181@xxxxxxxxxxxxxxxxxx> <20110501082717.5116e575@xxxxxxxxxxxxxx>
[ ... ]

>> * Can Linux MD do "abbreviated" read-modify-write RAID6
>> updates like for RAID5? [ ... ]

> No. (patches welcome).

Ahhhm, but let me dig a bit deeper, even if it may be implied in
the answer: would it be *possible*?

That is, is the double parity scheme used in MS such that it is
possible to "subtract" the old content of a page and "add" the
new content of that page to both parity pages?

[ ... ]

> The ideal config for a journalled filesystem is for put the
> journal on a separate smaller lower-latency device.  e.g. a
> small RAID1 pair.

> In a previous work place I had good results with:
>   RAID1 pair of small disks with root, swap, journal
>   Large RAID5/6 array with bulk of filesystem.

Sound reasonable, except that I am allergic to RAID5 (except in
two cases) and RAID6 (in general). :-), but would work equally
well I guess with RAID10 and its delightful MD implementation.

[ ... ]

Thanks for the information!

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