>>>>> "Ric" == Ric Wheeler <ricwheeler@xxxxxxxxx> writes:
>> so we're not depending on it zeroing blocks, we're just depending on
>> it advertising correctly whether or not it -does- zero.
If the relevant bits are set (ATA: DRAT and RZAT, SCSI: TPRZ) we'll set
the bdev's discard_zeroes_data flag.
[root@test ~]# lsscsi | grep SSD | awk '{ print $7 }'
/dev/sde
[root@test ~]# grep . /sys/block/sde/queue/discard_zeroes_data
1
The relevant ioctl is BLKDISCARDZEROES.
Ric> I think that ATA devices have historically not done this correctly,
I'm only aware of one drive that advertised RZAT and got it wrong. I
believe a firmware update fixed it.
Generally we assume that if the firmware writers go through the effort
of reporting things correctly then they have also implemented the
feature. We have quite a few sanity checks in place in libata so we
won't trigger if the firmware guys just put all ones in a word, for
instance. There are several things that need to line up for us to
actually set the discard flags.
PS. http://oss.oracle.com/~mkp/docs/linux-advanced-storage.pdf
--
Martin K. Petersen Oracle Linux Engineering
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