[Bug 237807] ZFS: ZVOL writes fast, ZVOL reads abysmal...

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=237807

--- Comment #3 from Nils Beyer <nbe at renzel.net> ---
(In reply to sigsys from comment #2)

the benchmark tool is completely sequential as it is a simple:

    dd if=/dev/zvol/morsleben-grube2/test of=/dev/null bs=1M
    ^C
    375390208 bytes transferred in 15.330245 secs (24486902 bytes/sec)


whereas the ZVOL has been filled before with random data using:

    dd if=/dev/urandom of=/dev/zvol/morsleben-grube2/test bs=1M
    ^C
    8544845824 bytes transferred in 77.684268 secs (109994546 bytes/sec)


"primarycache" and "secondarycache" are disabled for that ZVOL to get real read
speeds...


Now, I've followed your advice and tried two identical instances of "dd"
reading from the ZVOL. And you are right, I get twice the throughput: 40MB/s
now. It still is abysmal if you ask me, but okay...


The slow read throughput also exists via the iSCSI connection from a Windows
Server (the ZPOOL "morsleben-grub2" is a backup target). So I have to convince
the Microsoft iSCSI initiator somehow to create more concurrent reads.


Thank you for the pointers...

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