On Fri, Dec 12, 2014 at 02:54:24PM -0800, Robert White wrote: > I've seen it mentioned here that generally data extents are 1G and > metadata extents are 256M. > > Is that per-drive or per-stripe in the case of RAID0? > > That is, if I have data mode raid0 across N drives does the system > allocate one 1G extent on each drive making the full stripe > allocation N-gigs; or does it allocate 1/Nth(gig) on each drive > making the total new allocation 1G? > > Does the raid0 have any arity constraints (like how raid1 is always > arity-2)?
The 1 GiB (or 256 MiB for metadata) is the allocation unit. So for striped RAID levels (like 0, 10, 5, 6), the FS will allocate as many as it can across all the available devices, and stripe within those. Now on to your question -- the stripes within the allocation unit are 64 KiB in size, so the first 64k goes on the first device, the next 64k on the second device, and so on. The minimum stripe width (e.g. number of devices) is 2 for RAID-0, 4 for RAID-10, 2 for RAID-5 and 3 for RAID-6. Hugo. -- Hugo Mills | I get nervous when I see words like 'mayhaps' in a hugo@... carfax.org.uk | novel, because I fear that just round the corner is http://carfax.org.uk/ | lurking 'forsooth' PGP: 65E74AC0 | GRRM's UK editor
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