On 11/23/2009 03:37 PM, Alan Stern wrote:
On Mon, 23 Nov 2009, Ric Wheeler wrote:
Hi Alan,
One quick question - how robust is our support for write barriers over USB?
Specifically, I am looking to get together some testing with both
external USB and e-sata enclosure for various file systems and would be
very interested in helping make sure that this is well handled...
It's extremely robust -- the USB mass-storage driver has a maximum
command-queue length of 1!
With the coming of USB 3.0 and the UASP (USB-Attached SCSI Protocol)
specification this will change. I presume barriers will then be
implemented as the need arises.
Alan
What we need is to pass down cache flush commands (ATA_CACHE_FLUSH_EXT is what
flushed the cache for ATA/S-ATA devices). Even with a command-queue length of 1,
the write cache a USB connected s-ata drive would still loose data on power off
without this kind of support.
Ric
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