The choice of the unit of measure of storage is interesting to me because it's otherwise tought to measure price for performance.
remain agape at the price tag on high-end, supposedly high-performance,
storage systems. Connected by FibreChannel or gigabit Ethernet, that's a
limit of 400 and 110 MB/s, respectively. (Yes, I know of 8Gb/s FC and
10GE, but these are prohibitively expensive, if supported.Even
link-aggregated GigE practically tops out at 880MB/s) I'm thinking that
writes across 40 7200RPM disks could saturate an FC link, and it would
take fewer than 20 15k disks. Neither of these strikes me as impractical
or unusual sizes of storage arrays, even doubling those numbers for
RAID 1. More importantly, such arrays don't strike me as high
Particularly shocking is that a brand name "SAN"
solution of such a size would cost in the neighborhood of a quarter
million dollars and be at its performance limit. Granted, it might be
half that price without fancy management and replication software.
whereas the less fancy alternative, at one tenth to one fifth the cost,
would still be expandable from a performance standpoint. How much does
the Veritas database suite cost these days?
alternative, which I have implemented and benchmarked, is using
Serial-Attached SCSI (SAS) instead of FibreChannel and commodity SATA
disks instead of 10k or 15k spindles.Although it's not necessarily "SAN"
in the marketing sense, SAS readily supports multiple hosts per bus.
It's also typically implemented as 4x 300MB/s channels on one connector
for interfacing to expanders (a rough equivalent to FC switches). An x4
PCIe slot is actually the limiting throughput factor for one of these,
as each x1 lane is only 250MB/s. Even with RAID1, rolling my own array
would cost $25k (including labor), maybe double that for Dell brand
MD1000s. One could then spend twice again the same amount to get triple
the throughput on the same server(s), before running up against the
limit. Additional fanciness can be gained from 3rd-party storage
software vendors, especially in this economy, for under 6 figures.
for truly random I/O. For sequential I/O, such as for logs, the
situation is even more egregious: only 4 7.2k spindles would saturate a
(dedicated) FC link. If it's paired for redundancy, one would need a
second pair for the non-sequential, perhaps introducing some management
complexity, unless FC link aggregation becomes common enough to be
Another issue I've had come up in conversation is reliability and/or maintenance. This Usenix paper
belies the notion that SATA disks are any less reliable than others.
With a 3-6% annualized replacement rate, that's 2-5 disks per year, or
about 15% or 12 disks over 2.5 years, on an 80-disk array. I've actually
already included this (4 spares per 20 non-spares) in the $25k above.
between 2 and 3 years, you're going to have to bite the bullet, spend
another $25k for twice as much space, and migrate the old data, assuming
you're not already upgrading for other reasons. Woe is you. You'll just
have to resort to drowning your sorrows in the hundreds of grand you
saved, never mind the headache of shipping disks back and forth.
The Storage Emperor's new clothes are looking mighty skimpy, indeed.
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