FYI ... SDSS is using RAID 10 for the production servers, RAID 0 for
loadservers where performance trumps fault-tolerance. we found RAID 5
to be the worst of both worlds - not enough redundancy and much slower
write speeds.
ani
On Wed, 5 Jul 2006, Kirk D Borne wrote:
> Not a big issue, but just curious: which RAID level is
> recommended in these cases? e.g., what is BABAR using?
> how much disk redundancy does that imply?
>
>
> ----- Original Message -----
> From: Jacek Becla <[EMAIL PROTECTED]>
> Date: Wednesday, July 5, 2006 6:06 pm
> Subject: [LSST-data] number of disks needed to support db disk io
>
> > Hello,
> >
> > If we assume we have 2.3 TB of RAM and use it for common
> > indexes, (and force the queries to use these indexes even if
> > selectivity is high), then the number of disks needed = 4,400.
> >
> > DISKS, not disk heads(!). At the last telecon we (incorrectly)
> > assumed that this number is for disk heads. I spoke with our
> > local experts who confirmed that the number really is
> > for "disk head assemblies" (or spindles) because all disk heads
> > move together and there is one disk head assembly per spindle
> > (at least in most drives available today).
> >
> > I also checked with SLAC folks about number of disk
> > spindles to support BaBar, and at the moment it is ~2,000.
> > So the 1,000 disks for LSST proposed by the top-down estimate
> > seems rather low comparing to BaBar numbers. 4,400 seems
> > more realistic.
> >
> > The number of disks is highly dependent on data block size, e.g.:
> >
> > data number
> > block of
> > size disks
> > [KB]
> > ---------------
> > 16 43,200
> > 32 22,100
> > 64 11,700
> > 128 6,800 <-- I'd suggest to use either this
> > 256 4,400 <-- or this
> > 512 3,400
> > 1024 3,300
> >
> >
> > The small-ish disks ~2013 will probably be ~0.5 TB in size,
> > which would give us 2 PB of disk storage assuming 4,400 disks.
> > That sounds like a very good fit to what the database size
> > estimates suggest: 1.6 TB of disk space for DR2.
> > If we want to be less aggressive, we can pick the 128K data
> > block size.
> >
> > Assuming 12 disks per array, 2 unit high, 40 rack units per rack,
> > and back-to-back organization, we end up with 480 disks per rack,
> > and only 9 racks needed.
> >
> > The spreadsheet that supports these numbers:
> >
> > http://www.slac.stanford.edu/~becla/tmp/lsst_diskIO_estimates_v07.xls
> >
> > (I am still working on documentation and cleaning up
> > the spreadsheet.)
> >
> > BTW, all these numbers are for the usable disk space, we
> > should not forget about the RAID overheads, and disk spares.
> >
> > Jacek
> > _______________________________________________
> > LSST-data mailing list
> > [email protected]
> > http://www.lsstmail.org/mailman/listinfo/lsst-data
> >
> _______________________________________________
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>
--
Aniruddha R. Thakar, Research Scientist
Center for Astrophysical Sciences, JHU, Bloomberg 375
3701 San Martin Drive, Baltimore MD 21218-2695
410-516-4850, Fax: 410-516-5096
[EMAIL PROTECTED], http://www.sdss.jhu.edu/~thakar
-----------------------------------------------------------------------
In some circumstances, the refusal to be defeated is a refusal to be
educated. [Margaret Halsey]
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