Proposal to add a new modifier to SCSI Write commands. -Reply
Jeff Blaalid, CMD Technology, (714) 470-3155
BLAALID at cmd.com
Tue Dec 3 09:01:52 PST 1996
* From the SCSI Reflector (scsi at symbios.com), posted by:
* "Jeff Blaalid, CMD Technology, (714) 470-3155" <BLAALID at cmd.com>
> The real goal ...
> "Mark the block" [to generate] "a CHECK
> CONDITION when read". "without having to
> interfere with the data payload or ecc areas
>Thought 1: Totally cool idea.
>Thought 2 ...
>One of the toy projects I might finish if I were
>independently wealthy would be to see how much of
>SCSI I could implement as a layer on the host
>side. In particular by branching on x12 Inquiry
>data to cope with the vagaries of specific
>Doesn't this idea boil down to a bitmap of all
>LBA's, stored outside of the LBA's, which the host
>For LBA's that have a bit set, some i/o layer
>generates a CHECK CONDITION when the LBA is
>otherwise read OK?
Yes - the final goal is to create tables on the
disks (hiding blocks from the host) and placing
meta-data that can describe the "state of the data"
on the disk.
But, the tables are very large (9+GBdisks) and
difficult to look at for each i/o operation.
EG: 1 bit/block for a 9GB disk
18874368 block 1 bit/block
--------------- * ------------ = 4608 blocks or ~2.3 MB
512 bytes/block 8 bits/byte
Now this isn't much of a "storage loss" to the user but try
maintaining this table for 42 devices or more from a RAID controller
or host. The other problem is searching/indexing
this table for each and every operation. It isn't difficult but
gets placed in the most critical execution path that will have
direct impact on QIO and spiral performance.
Today many disk controllers (RAID) maintain the metadata on the
disk as you suggest but use the READL/WRITEL operations to create
ECC errors on the drive - thus a FLAG to tell them to check their
The goal of the "reactive data" is to have
the drive "tell you to look" at your tables!
Keep in mind that this approach is NOT limitted to "FORCED BAD" but
can be used to indicate all kinds of things - like XOR PARITY is not
Finally, the ultimate solution would be a "FORMAT" specified HOST HEADER
overhead that would allow for this to be larger that ONE BIT. The
drive could (if enabled) generate CHECK CONDITION when any of the BITS
are non-zero. This would allow the hosts to add whatever
they desired to each block. Also, a READ modifier to do
continous reads including the "HOST HEADER" information at the
front of each block would be very nice.
But, I would be happy with just the "reactive data" bit today!!
Thanks for the great discussion - This is one of the biggests gaps
in the SCSI storage specification. When controllers own'ed the formats
these were the tricks used to insure availability and accuracy of
the USER'S data.
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