Bill Davidsen wrote: > Paul Clements wrote: >> Georgi Alexandrov wrote: >> >>> Generally with the healthy array I'm getting the write performance of >>> the SATA disk alone (in terms of requests/sec issued to the disk and >>> bytes/sec written). The SATA disk is obviously a bottleneck even with >>> the write-behind option set(2). >> >> write-behind can help with two things: >> >> 1) overcoming latency (say one disk is on the network -- it may be the >> same speed as the source disk, but it takes longer round-trip for each >> I/O to complete) >> >> 2) temporary slowness of a device (say at a peak in I/O) -- the queue >> can temporarily hide the slowness of the secondary disk, but this >> won't last very long -- if writes continue at a pace faster than the >> disk can handle (i.e., the queue gets filled) then the array drops >> back to non-write-behind behavior >> > At least with write-mostly all of the capacity is going into saving > data, not serving data. But as you note below if the writes are > happening at a rate faster than the device can support it will be a > bottleneck. <snip> Well, at least write-mostly is suitable for reading from the SSD disk only in a setup like mine. If writes get really problematic maybe it's better to consider a SSD-only solution. -- regards, Georgi Alexandrov key server - pgp.mit.edu :: key id - 0x37B4B3EE Key fingerprint = E429 BF93 FA67 44E9 B7D4 F89E F990 01C1 37B4 B3EE
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