Re: [Qemu-devel] [PATCH 1/2 v1] blkdrv: Add queue limits parameters for sg block drive

[Stefan Hajnoczi]

On Tue, Aug 21, 2012 at 10:41 AM, Cong Meng <address@hidden> wrote:
>
>
> On Tue 21 Aug 2012 04:48:17 PM CST, Paolo Bonzini wrote:
>>
>> Il 21/08/2012 10:23, Cong Meng ha scritto:
>>>
>>> +static void sg_get_queue_limits(BlockDriverState *bs, const char
>>> *filename)
>>> +{
>>> +    DIR *ffs;
>>> +    struct dirent *d;
>>> +    char path[MAXPATHLEN];
>>> +
>>> +    snprintf(path, MAXPATHLEN,
>>> +             "/sys/class/scsi_generic/sg%s/device/block/",
>>> +             filename + strlen("/dev/sg"));
>>> +
>>> +    ffs = opendir(path);
>>> +    if (!ffs) {
>>> +        return;
>>> +    }
>>> +
>>> +    for (;;) {
>>> +        d = readdir(ffs);
>>> +        if (!d) {
>>> +            return;
>>> +        }
>>> +
>>> +        if (strcmp(d->d_name, ".") == 0 || strcmp(d->d_name, "..") == 0)
>>> {
>>> +            continue;
>>> +        }
>>> +
>>> +        break;
>>> +    }
>>> +
>>> +    closedir(ffs);
>>> +
>>> +    pstrcat(path, MAXPATHLEN, d->d_name);
>>> +    pstrcat(path, MAXPATHLEN, "/queue/");
>>> +
>>> +    read_queue_limit(path, "max_sectors_kb", &bs->max_sectors);
>>> +    read_queue_limit(path, "max_segments", &bs->max_segments);
>>> +    read_queue_limit(path, "max_segment_size", &bs->max_segment_size);
>>> +}
>>
>>
>> Using /sys/dev/block or /sys/dev/char seems easier, and lets you
>> retrieve the parameters for block devices too.
>>
> what do you mean with "block devices"?   Using "/dev/sda" instead of
> "/dev/sg0"?
>
>
>> However, I'm worried of the consequences this has for migration.  You
>> could have the same physical disk accessed with two different HBAs, with
>> different limits.  So I don't know if this can really be solved at all.
>>
> I know little about qemu migration now.  The pending scsi commands will be
> saved and
> transfered to remote machine when starting migration?

Passthrough is already a migration blocker if both hosts do not have
access to the same LUNs.

When both hosts do have access to the same LUNs it's possible to
extract the block queue limits (using sysfs) and compare them.

Today you can start QEMU with different image files on both hosts.
Migration will appear to work but the disk image on the destination
host could be junk.  This is a similar case, I don't see a problem
except that there should be a safety check (maybe at the libvirt
level) to make this safe.

Stefan