RAID 5, ~20% of extra disk space needed
Jacek
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
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