On Fri, Dec 12, 2014 at 03:25:19PM -0800, Robert White wrote: > On 12/12/2014 02:59 PM, Hugo Mills wrote: > >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. > > > > [So to check my understanding, and just sticking to RAID-0 data only]. > > So for RAID-0 data on 5 drives with ample space, the expected > outcome of allocating more data space is 5GiB, one 1GiB allocated on > each drive.
Correct. > If one drive is too full (say it was smaller) and didn't have 1G of > contiguous space available, the allocation would simply fail. No, it would allocate on the remaining 4 devices instead, with a total of 4 GiB of space. The allocation in these cases is the maximum feasible, not precisely the number of devices. > The net effect is to create an association of allocations, one on > each available drive that had "enough space", each of which will > contribute exactly 1GiB to the association. Yes. > So every time the data > space allocation expands its going to expand by N-GiB total on an > N-drive data=raid0 system. Not necessarily -- if one device is already full (because it's smaller), then the number of devices will decrease as appropriate, down to the minimum of 2. > Since data and metadata are separate you can end up being "out of > space" for big files but still be able to create files small enough > to fit into the metadata with the inode. Yes, but this isn't related to the number of devices in striped RAID allocations. > Am I correct? Partially. :) Hugo. -- Hugo Mills | "How deep will this sub go?" hugo@... carfax.org.uk | "Oh, she'll go all the way to the bottom if we don't http://carfax.org.uk/ | stop her." PGP: 65E74AC0 | U571
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