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data switchs su,sw and sunit,swidth

To: Linux-Xfs <linux-xfs@xxxxxxxxxxx>
Subject: data switchs su,sw and sunit,swidth
From: Linda Walsh <xfs@xxxxxxxxx>
Date: Sat, 18 Jul 2009 17:54:29 -0700
User-agent: Thunderbird (Windows/20090605)
Resending this as I never saw it show up on the list  (sent out
yesterday) (while other the messages came back in under 15 minutes or so)...

Have started to use RAID on a few of my disks and forgot about the
xfs 'su*sw' and 'sunit*swidth' options.

From what I get in reading the manpage, 'su' is used with 'sw' and
'sunit' is used with 'swidth'?

The RAID controller in one of my machines uses a "strip element" size, expressed in bytes, allowed values seem to be limited to powers of 512*2^*[1..11] (512B up to 1MB) (though as previously noticed, although
xfs's manpages claims to allow one expresses sizes with the unit 'm', it
only permits .25m (256k), I guess I never tried seeing if the command line would take floating point ;^) ).

I believe 'su' would be set to the 'strip element size' (in k or m).

Then, for RAID 1 (mirror) would 'sw'==1?  Would setting the  su/sw  value
for a RAID 1 actually matter in any way? Ie, technically -- it would fill in numbers for OS book-keeping, but wouldn't change anything in terms of performance or layout, vs. 'physically' -- where it could change
disk layout or performance?

At RAID 0, I'd guess  sw==2?

In RAID 5, would it be   sw  == #Disks-1?  So even w/6 disks, it still only
uses 1 disk for parity and sw == 5?
I wonder what becomes a max-safe RAID 5 size? (or is the number of parity
disks a settable option with RAID 5?)


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