On 14-07-18 07:41 AM, James Bottomley wrote:
On Fri, 2014-07-18 at 17:17 +0000, Elliott, Robert (Server Storage)
wrote:
From: James Bottomley [mailto:jbottomley@xxxxxxxxxxxxx]
On Fri, 2014-07-18 at 00:51 +0000, Elliott, Robert (Server Storage)
wrote:
...
Also, in both sd_setup_flush_cmnd and sd_sync_cache:
cmd->cmnd[0] = SYNCHRONIZE_CACHE;
cmd->cmd_len = 10;
SYNCHRONIZE CACHE (16) should be favored over SYNCHRONIZE
CACHE (10) unless SYNCHRONIZE CACHE (10) is not supported.
(sorry - meant "unless ... 16 is not supported")
Yes, I guessed that?
For what reason. We usually go for the safe alternatives, which is 10
byte commands because they have the widest testing and greatest level of
support. We don't do range flushes currently, so there doesn't seem to
be a practical different. If we did support range flushes, we'd likely
only use SC(16) on >2TB devices.
James
A goal of the simplified SCSI feature set idea is to drop all the
short CDBs that have larger, more capable equivalents - don't carry
READ 6/10/12/16 and SYNCHRONIZE CACHE 10/16, just keep the 16-byte
versions. With modern serial IU-based protocols, short CDBs don't
save any transfer time. This will simplify design and testing on
both initiator and target sides. Competing command sets like NVMe
rightly point out that SCSI has too much legacy baggage - all you
need for IO is one READ, one WRITE, and one FLUSH command.
But that's not relevant to us. This is the problem of practical vs
standards approaches. We have to support older and buggy devices. Most
small USB storage sticks die if they see 16 byte CDB commands because
their interpreters. The more "legacy baggage" the standards committee
dumps, the greater the number of heuristics OSs have to have to cope
with the plethora of odd devices.
That's why SBC-3 ended up with these warning notes for all the
non-16 byte CDBs:
NOTE 15 - Migration from the SYNCHRONIZE CACHE (10) command to
the SYNCHRONIZE CACHE (16) command is recommended for all
implementations.
If the LBA field in SYNCHRONIZE CACHE went obsolete, then maybe
SYNCHRONIZE CACHE (10) would be kept instead of (16), but that
field is still present. So, (16) is the likely survivor.
OK, but look at it from our point of view: The reply above justifies
why we prefer 10 byte CDBs over 16. If the standards body ever did
remove SC(10) completely, then we'd have to have yet another heuristic
to recognise devices that don't support SC(10), but until that day,
using SC(10) alone works in all cases, so is the better path for the OS.
If you could, please get the standards body to recognise that the more
command churn they introduce (in the name of rationalisation or
whatever), the more problems they introduce for Operating Systems and
the more likelihood (because of different people reading different
revisions of standards) that we end up with compliance bugs in devices.
From the term: "feature sets" I'm guessing T10 will follow what T13
does and have something like a VPD page with descriptors of feature
sets supported. Each set has mandatory and optional commands,
perhaps a similar categorization of mode and VPD pages as well. Such
a "clean slate" for SCSI would make it simpler in the future, at
least for what to put on the fast path. Perhaps some legacy
support could be pushed to the user space.
For many technical areas "legacy" is a derogatory term, but
not necessarily for storage!
Doug Gilbert
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