Dean S. Messing wrote:
Bill Davidsen wrote:
: Dean S. Messing wrote:
: > Again, I don't get these speeds. Seq. reads are about
: > 170% of the average of my three physical drives if I turn up
: > the look-ahead. Then random access reads drops to slightly less
: > than my slowest drive.
: >
: As nearly as I can tell, Dean was talking about RAID-10 at that point (I
: also suggested that) which you haven't tried.
I was talking about the three drive RAID-5 on which I ran bonnie++ measurements.
I have not (yet) tried RAID-10.
: For small numbers of
: drives, assume the read speed will be (N - 1) * S for large sequential
: read, using RAID-10. Where S is the speed of a single drive. Random read
: depends on so many things I can't begin to quantify them in anything
: less than a full white paper, but for a single thread assume somewhere
: around S and aggregate (N - 1) * S again. Writes depend a lot on system
: tuning, stripe size, stripe_cache_size, chunk size, etc. Fortunately the
: best way to boost write speed is to have lots of memory and let the
: kernel buffer.
How does one "let the kernel buffer"? (I have plenty of memory for
most things.) I know about "write-back" vs. "write-through" to reduce
the write asymmetry of RAID-5. Is this what you mean by a kernel
buffer?
Just by having adequate memory you will get kernel buffering (unless you
use fsync or similar), and performance goes up if you increase your
stripe_cache_size, although you hit diminishing returns on that
somewhere between 8-32MB.
: Finally, when you create your ext filesystem, think of:
: - ext2 - no journal
: - noatime mounts to avoid journal writes
Please try this before you reach any conclusions. Doing measurements on
a filesystem instead of raw raid arrays adds bottlenecks.
: - manually make the journal file *large* to spread head motion over drives
: - consider moving journal file to a dedicated device (that old 20GB
: PATA drive?)
: - use the ext3 "stride" tuning stuff (I'm quantifying that in the next
: ten days).
:
: Or just make a RAID-10 "far" array and stop agonizing over this stuff,
: there is no config which is best for everything, you must realize "fast,
: cheap, reliable - pick two" is the design paradigm of RAID, and the more
: you optimize for one usage pattern the more you impact some other.
Dean
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--
bill davidsen <davidsen@xxxxxxx>
CTO TMR Associates, Inc
Doing interesting things with small computers since 1979
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