Anantha N. Srirama wrote On 01/08/07 13:04,:
Our setup:
- E2900 (24 x 96); Solaris 10 Update 2 (aka 06/06)
- 2 2Gbps FC HBA
- EMC DMX storage
- 50 x 64GB LUNs configured in 1 ZFS pool
- Many filesystems created with COMPRESS enabled; specifically I've one that is
768GB
I'm observing the following puzzling behavior:
- We are currently creating a large (>1.4TB) and sparse dataset; most of the
dataset contains repeating blanks (default/standard SAS dataset behavior.)
- ls -l reports the file size as 1.4+TB and du -sk reports the actual on disk
usage at around 65GB.
- My I/O on the system is pegged at 150+MB/S as reported by zpool iostat and
I've confirmed the same with iostat.
This is very confusing
- ZFS is doing very good compression as reported by the ratio of on disk versus as reported size of the file (1.4TB vs 65GB)
- [b]Why on God's green earth am I observing such high I/O when indeed ZFS is
compressing?[/b] I can't believe that the program is actually generating I/O at
the rate of (150MB/S * compressratio).
Any thoughts?
One possibility is that the data is written synchronously (uses O_DSYNC,
fsync, etc), and so the ZFS Intent Log (ZIL) will write that uncompressed
data to stable storage in case of a crash/power fail before the txg
is committed.
Neil.
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