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 > _______________________________________________ LSST-data mailing list [email protected] http://www.lsstmail.org/mailman/listinfo/lsst-data
