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Re: SATA write performance problems (on HP MicroServer)
On Tue, 1 Jan 2013, Edgar Fuß wrote:
Does your drive have write-caching disabled?
Yes, I think so. But a) why should it be so slow without write cacheing and b)
doesn't write cacheing call for corrupted file systems in case of a crash?
a) No write cache means the drive will not complete the write request
until it has written all the data on the disk. This may well require on
average, waiting for 1/2 the disk rotation time, and in the case of
sequential write, it may well have to wait a full rotation for each write.
Assuming a 7200 RPM drive, that works out to be 120 writes per second,
which matches the number of I/Os you see.
b) I wouldn't think so. In fact, it may possibly reduce the likelyhood
of a corrupted file system after a crash. With write caching enabled, you
may get more of the disk buffers written to the drive cache rather than
sitting in memory. Anything in memory will be gone after a crash that is
unable to flush any buffers. If the data gets to the drive cache, it
should make it to the disk eventually. Now a crash due to a power failure
is a different story. Anything in the drive's cache would be lost (but
then so would anything sitting in memory). Write caching could also
disrupt any ordered writes done by the OS, which may get data written to
the disk in an order that could lead to corruption.
Michael L. Hitch mhitch%montana.edu@localhost
Information Technology Center
Montana State University Bozeman, MT USA
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