Maurilio,
what is the correct path to use with iscsitgtd when using a zvol as
backing storage?
Using the dsk devices introduces the Solaris buffer cache into the I/O
path, and given that ZFS also adds caching to zvols, it raises the
performance anomaly of having a cache in front of a cache. Have two
layers of caching, especially when there is no coordination between
the caches, causes higher cache miss rates, and often sporadic I/O
performance.
Use the rdsk device to remove the Solaris buffer cache.
I've seen examples and references which sometimes use /dev/zvol/
dsk/... and sometimes /dev/zvol/rdsk/...
I've also read that using the /dsk/ path makes iscsitgtd to try to
allocate as much ram as the size of the zvol.
Prior to build snv_92, for all dsk devices the iscsitgt would use the
mmap(2) routines to map files or devices into Solaris memory as
needed. This was compounded by the fact that the daemon would then
traverse the entire iSCSI LU, zeroing out the device blocks to prevent
an accessor from reading unwritten data. The combination of mmap,
zeroing our the device, ZFS's redirect on write, and ZFS's use of
kernel memory, introduced very poor I/O performance.
So, I'd like to know an official answer to this question which I'm
not able to find on man pages, blogs and so on.
rdsk
Thanks a lot.
Maurilio.
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