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    SubjectRe: sg regression in 2.6.16-rc5
    Kai Makisara wrote:
    > On Tue, 28 Feb 2006, Douglas Gilbert wrote:
    >>Mark Rustad wrote:
    >>>We have encountered some kind of sg regression with kernel 2.6.16-rc5
    >>>relative to 2.6.15. We have a small program that demonstrates the
    >>>failure. On 2.6.15 it produces the output:
    >>>Alloced dataptr 0 -> 0xb7d07008
    >>>IOS: 0
    >>>ios 100
    >>>indicating that it did 100 operations successfully. On 2.6.16-rc5, it
    >>>produces the output:
    >>>Alloced dataptr 0 -> 0xa7d10008
    >>>SG_IO ioctl error 12 Cannot allocate memory
    >>>ios 0
    >>>indicating that it did 0 operations successfully. This program is
    >>>attempting to do 1MB reads on a SCSI device.
    >>You can stop right there with the 1 MB reads. Welcome
    >>to the new, blander sg driver which now shares many
    >>size shortcomings with the block subsystem.
    >>In lk 2.6.15 the sg driver (and the st driver) did its
    >>own scatter gather list allocations. The sg driver
    >>used 32 KB segments (8 times the normal page size)
    >>in each scatter gather element. The maximum number
    >>of scatter gather elements depends on the LLD but
    >>can be no more than 256. That meant the sg driver
    >>allowed a maximum single IO size of 8 MB. There was
    >>also a define in sg.h (SG_SCATTER_SZ and it is still
    >>there) that allowed the 32KB per segment to be increased
    >>allowing larger single command transfers (then 8 MB).
    > This is still possible but it needs some changes to most SCSI HBA drivers.
    > The big requests are split into bios supporting 256 pages. For 4 kB pages,
    > this limits i/o to 1 MB. The scsi_execute_async() path used by st and sg
    > can chain bios and this enables large request at the ULD level. At lower
    > level, the request consists of pages and now we hit the s/g list maximum
    > length _unless_ the HBA driver enables clustering. In this case the
    > adjacent pages are coalesced and the large requests fit into the HBA s/g
    > limits. Well, now we hit another limit: the max_sectors default for SCSI
    > drivers is 1024 and this limits requests to 512 kB _unless_ the HBA driver
    > increases max_sectors.
    > The aic79xx driver enables clustering but does not increase max_sectors.
    > This makes the maximum request size 512 kB. If it is possible to set
    > .max_sectors = 0xFFFF,
    > in linux/drivers/scsi/aic7xxx/aic79xx_osm.c without breaking the driver,
    > this should enable requests up to 8 MB - 256 B. (I don't have the hardware
    > to test this.)
    > Several SCSI HBA drivers currently have similar problems.

    I applied the above changes to my scsi_debug (plus
    extended .sg_tablesize to SG_ALL (it was 64)) and
    my single command READs topped out at 4 MB exactly
    (bs=512 bpt=8192). When I tried bpt=8193 the
    SG_IO ioctl (via a sg device) yielded ENOMEM which
    is much more informative than EIO.

    That is an improvement.

    Doug Gilbert
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