linux-ext4 - What to put for unknown stripe-width?

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Message-id: <4E786B53.5020407@shiftmail.org>
Date:	Tue, 20 Sep 2011 12:30:43 +0200
From:	torn5 <torn5@...ftmail.org>
To:	linux-ext4@...r.kernel.org
Subject: What to put for unknown stripe-width?

Hello there,
I am planning some ext4 filesystems which are currently in (LVM over) a 
RAID array having:

stride=1MiB   stripe-witdth=4MiB

BUT ... their RAID could be enlarged with more disks in the future 
and/or they are likely to be moved around live (leveraging LVM, which I 
will align) to other RAIDs which also will have stride=1MiB but unknown, 
as of now, stripe-width.
That's on HDDs, with platters.

What do you suggest for stripe-width?
I don't really know how ext4 works, unfortunately. But I think the 
answer should be among the following values:

-  a) 60MiB: so to be exact multiple of most stripe-widths, in 
particular when number of data disks is any of: 
1,2,3,4,5,10,12,15,20,30,60 . I expect some longer-than-normal seeks 
with HDD heads with 60MiB though.

-  b) 7MiB or 11MiB: (prime numbers so *not* likely multiple of most 
stripe-widths) so to likely see data eventually spreaded equally on the 
various disks (maybe?). That's the opposite reason as (a), so one of 
these two must be wrong.

-  c) 1MiB so to be exact *divisor* (and not multiple) of all possible 
number of disks. This is wrong, isn't it?

-  d) use current optimal of 4MiB, then use tune2fs to alter 
stripe-width when the underlying stripes are changed. This should be 
fine for new writes, but I am not sure what is the impact on reads of 
old data.

Thanks for your help
T5
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