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Re: [Libvir] [PATCH] Block device and network stats (version 2)



Daniel P. Berrange wrote:
On Mon, Aug 20, 2007 at 04:31:26PM +0100, Richard W.M. Jones wrote:
This patch adds block device and network stats. The main changes over the previous version are:

* Remote support.
* Change Xen network interface names to have the form "vif<domid>.<n>"[1].

Discussions about the previous version may be found starting here:
https://www.redhat.com/archives/libvir-list/2007-August/thread.html#00064

I have left use of stdint.h / int64_t, since it wasn't clear to me what conclusion people had arrived at.

Personally I'm for using long long, since its consistent with the other
existing APIs using 64 bit quantities. They're both standards so there's
no much of a reason to favour one over the other.

I favour int64_t, because I want 64 bit integers (signed in this case because I want the special -1 value). C doesn't specify a maximum width for "long long".

I notice the Xen impl of the block stats only fills in the rd_req and wr_req
fields, not the rd_bytes and wr_bytes fields. Are requests always fixed at
512 bytes in size ? If so, should be just junk those fields and only return
data in terms of the bytes (other units can be calculated as needed). As a
point of reference libstatgrab only returns bytes read/written for disks.

I assumed that Xen requests were variable in size ... That is based on my reading of this code from blkif.h:

#define BLKIF_MAX_SEGMENTS_PER_REQUEST 11

struct blkif_request_segment {
    grant_ref_t gref;        /* reference to I/O buffer frame        */
    /* @first_sect: first sector in frame to transfer (inclusive).   */
    /* @last_sect: last sector in frame to transfer (inclusive).     */
    uint8_t     first_sect, last_sect;
};

struct blkif_request {
    uint8_t        operation;    /* BLKIF_OP_???                         */
    uint8_t        nr_segments;  /* number of segments                   */
    blkif_vdev_t   handle;       /* only for read/write requests         */
    uint64_t       id;           /* private guest value, echoed in resp  */
    blkif_sector_t sector_number;/* start sector idx on disk (r/w only)  */
    struct blkif_request_segment seg[BLKIF_MAX_SEGMENTS_PER_REQUEST];
};
typedef struct blkif_request blkif_request_t;

But to be honest I don't know (the source to Xen I have here still uses the sparse tree, making it impossible to find the actual source for the Linux drivers ...).

If it is true that requests have a fixed size, then returning bytes _also_ would be good, but I suspect that we should also still return requests as well since (a) it costs virtually nothing to do so, and (b) it might be interesting to find the average size of each request, since lots of small requests are likely to be less efficient than small numbers of large requests.

The Xen impl as coded only works for disks named xvdN, because the code for
calculating device ID assumes xvdN device numbering scheme:

    device = 202 * 256 + minor;

I know this is all utterly horrific, but we need to apply same logic as
used in XenD for sdNNN and hdNNN :-( For sdNNN based disks it seems to be

    8 * 256 + 16 * (ord value of disk letter ) + partition number

For hdNNN based disks it seems to be
    ide major number corresponding to disk letter * 256 + minor number
    as calculated from partition numbers.

OK ... I've never seen Xen guests with sdX/hdX devices.  Are they common?

The interface stats look OK to me. The impl which parses /proc/net/dev
though could be shared with the QEMU driver - only the device name needs
to be different between them - QEMU will be vnetXXX - we have the actual
dev name when we create teh TAP device, but don't bother to save it anywhere
from the looks of things.

I left the explicit structure size parameter to allow for future extensibility.

Good plan.

Regards,
Dan.

Rich.

--
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