On 10/22/2010 01:03 PM, Martin K. Petersen wrote:
"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
Hi Martin,
I think that the T13 people are still reworking (or have just finished?) the
spec. For example,
http://www.t13.org/documents/UploadedDocuments/docs2010/e09158r2-Trim_Clarifications.pdf
has more detail on how to interpret those bits. I certainly cannot claim to
have followed all of this recently, but do recall that early on the ATA devices
specifically had various behaviors that caused concern.
Ric
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