[Lustre-discuss] how the lustre distribute data among disks within one OST

[Christopher J. Morrone]

In that case, it is the question part that I do not understand. :)  What 
is "stripe 0,4", why could it be "closer" then "stripe 0,2"?  In your 
example, 0, 2, and 4 are all in the same place.

If you file is striped over 2 OSTs, then essentially what happens behind 
the scenes is that there are two files, one on each OST.  But Lustre 
hides that from you, as a user.  Lustre basically does modulo operations 
to translate a file offset from the file that it presents to the user, 
into which ost and offset into said ost's file to use.

Does that help at all?

Chris

On 06/13/2013 02:58 PM, Jaln wrote:
> Oh, I mean there is one file, for example 6 MB, the stripe size is 1MB,
> and only 2 OST,
> then the file will be divided into 6 stripes, denoted as stripe 0,1,2,3,4,5.
> the distribution on the 2 OST  would be stripe 0,2,4 on OST0, stripe
> 1,3,5 on OST1.
>
> Jaln
>
>
> On Thu, Jun 13, 2013 at 2:54 PM, Christopher J. Morrone
> <morrone2 at llnl.gov <mailto:morrone2 at llnl.gov>> wrote:
>
>     I think you may be confused about what a stripe is in Lustre.  If
>     there are only 2 OST, then you can only stripe a file across 2.
>
>     Or maybe I don't understand your terminology.  I don't know what you
>     mean by "0,4" and "0,2".
>
>
>     On 06/13/2013 02:38 PM, Jaln wrote:
>
>         if I have 6 stripes, 2 OST, using round-robin striping,
>         stripe 0,2,4 will be on OST0,
>         stripe 1,3,5 will be on OST1,
>         Do you guys have any idea about what will be the difference of
>         accessing
>         stripe 0,4 vs stripe 0,2?
>         stripe 0, 2 seems to be closer than 0,4, or the lustre will do
>         some intelligent work?
>
>         Jaln
>
>
>         On Thu, Jun 13, 2013 at 10:22 AM, Christopher J. Morrone
>         <morrone2 at llnl.gov <mailto:morrone2 at llnl.gov>
>         <mailto:morrone2 at llnl.gov <mailto:morrone2 at llnl.gov>>> wrote:
>
>              On 06/13/2013 05:19 AM, E.S. Rosenberg wrote:
>               > On Thu, Jun 13, 2013 at 3:09 AM, Christopher J. Morrone
>               > <morrone2 at llnl.gov <mailto:morrone2 at llnl.gov>
>         <mailto:morrone2 at llnl.gov <mailto:morrone2 at llnl.gov>>> wrote:
>               >> Lustre does not  manage the individual disks.  I sits
>         on top of a
>               >> filesystem, either ldiskfs(basically ext4) or zfs (as
>         of Lustre
>              2.4).
>               > Is ZFS the recommended fs, or just an option?
>               > Doesn't ZFS suffer major performance drawbacks on linux
>         due to it
>               > living in userspace?
>               > Thanks,
>               > Eli
>
>              LLNL (Brian Behlendorf) ported ZFS natively to Linux.  We
>         are not using
>              the FUSE (userspace) version.  You can find it at:
>
>         http://zfsonlinux.org
>
>              ZFS is one of the two backend filesystem options for
>         Lustre, as of
>              Lustre 2.4.  2.4 is the first Lustre release that fully
>         supports using
>              ZFS.  Here at LLNL we are using it on our newest, and
>         largest at 55PB,
>              filesystem.
>
>              Chris
>
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