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

[Cong Meng <mc@linux.vnet.ibm.com>]

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?

Cong.

> Paolo
>