On 2013-02-04 15:52, Edward Ned Harvey
I noticed that sometimes I had terrible rates with < 10MB/sec. Then
later it rose up to < 70MB/sec.
Are you talking about scrub rates for the complete scrub? Because if you sit
there and watch it, from minute to minute, it's normal for it to bounce really
low for a long time, and then really high for a long time, etc. The only
measurement that has any real meaning is time to completion.
To paraphrase, the random IOs on HDDs are slow - these are multiple
reads of small blocks dispersed on the disk, be it small files or
copies of metadata or seeks into the DDT. Fast reads are large
sequentially stored files, i.e. when a scrub hits an ISO image or
a movie on your disk, or a series of smaller files from the same
directory than happened to be created and saved in the same TXG
or so, and their userdata was queued to disk as a large sequential
blob in a "coalesced" write operation.
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