On Sat, Aug 20, 2011 at 8:38 AM, Jeremy Fitzhardinge <jeremy@xxxxxxxx> wrote:
> On 08/18/2011 02:34 AM, Li Dongyang wrote:
>> This adds the BLKIF_OP_TRIM for blkfront and blkback, also 2 flags telling
>> us the type of the backend, used in blkback to determine what to do when we
>> see a trim request.
>> Part of the patch is just taken from Owen Smith, Thanks
>>
>> Signed-off-by: Owen Smith <owen.smith@xxxxxxxxxx>
>> Signed-off-by: Li Dongyang <lidongyang@xxxxxxxxxx>
>> ---
>> include/xen/interface/io/blkif.h | 21 +++++++++++++++++++++
>> 1 files changed, 21 insertions(+), 0 deletions(-)
>>
>> diff --git a/include/xen/interface/io/blkif.h
>> b/include/xen/interface/io/blkif.h
>> index 3d5d6db..b92cf23 100644
>> --- a/include/xen/interface/io/blkif.h
>> +++ b/include/xen/interface/io/blkif.h
>> @@ -57,6 +57,19 @@ typedef uint64_t blkif_sector_t;
>> * "feature-flush-cache" node!
>> */
>> #define BLKIF_OP_FLUSH_DISKCACHE 3
>> +
>> +/*
>> + * Recognised only if "feature-trim" is present in backend xenbus info.
>> + * The "feature-trim" node contains a boolean indicating whether barrier
>> + * requests are likely to succeed or fail. Either way, a trim request
>
> Barrier requests?
hm, I wonder the same, seems it's a copy & paste mistake,
the BLKIF_OP_TRIM part is taken from Owen's patch back in Jan 2011:
http://lists.xensource.com/archives/html/xen-devel/2011-01/msg01059.html
>
>> + * may fail at any time with BLKIF_RSP_EOPNOTSUPP if it is unsupported by
>> + * the underlying block-device hardware. The boolean simply indicates
>> whether
>> + * or not it is worthwhile for the frontend to attempt trim requests.
>> + * If a backend does not recognise BLKIF_OP_TRIM, it should *not*
>> + * create the "feature-trim" node!
>
> Is all this necessary? What happens if guests just send OP_TRIM
> requests, and if the host doesn't understand them then it will fails
> them with EOPNOTSUPP? Is a TRIM request ever anything more than a hint
> to the backend that certain blocks are no longer needed?
that won't happen: we only mark the queue in the guest has TRIM if
blkback tells blkfront
via xenstore. if we don't init the queue with TRIM in guest, if guest
send OP_TRIM,
it gonna fail with ENONOTSUPP in the guest's block layer, see
blkdev_issue_discard.
and yes, trim is just a hint, the basic idea is forward the hint to
phy dev if it has trim, or
punch a hole to reduce disk usage if the backend is a file.
and this comment is taken from Owen, I think he could give sth here.
>
>> + */
>> +#define BLKIF_OP_TRIM 5
>> +
>> /*
>> * Maximum scatter/gather segments per request.
>> * This is carefully chosen so that sizeof(struct blkif_ring) <= PAGE_SIZE.
>> @@ -74,6 +87,11 @@ struct blkif_request_rw {
>> } seg[BLKIF_MAX_SEGMENTS_PER_REQUEST];
>> };
>>
>> +struct blkif_request_trim {
>> + blkif_sector_t sector_number;
>> + uint64_t nr_sectors;
>> +};
>> +
>> struct blkif_request {
>> uint8_t operation; /* BLKIF_OP_??? */
>> uint8_t nr_segments; /* number of segments */
>> @@ -81,6 +99,7 @@ struct blkif_request {
>> uint64_t id; /* private guest value, echoed in resp */
>> union {
>> struct blkif_request_rw rw;
>> + struct blkif_request_trim trim;
>> } u;
>> };
>>
>> @@ -109,6 +128,8 @@ DEFINE_RING_TYPES(blkif, struct blkif_request, struct
>> blkif_response);
>> #define VDISK_CDROM 0x1
>> #define VDISK_REMOVABLE 0x2
>> #define VDISK_READONLY 0x4
>> +#define VDISK_FILE_BACKEND 0x8
>> +#define VDISK_PHY_BACKEND 0x10
>
> What are these for? Why does a frontend care about these?
they are used for the backend driver to decide what to do when we get
a OP_TRIM, if it's phy then forward the trim,
or punch a hole in the file, Jan also mentioned this is not good,
gonna find another place for the flags, thanks
>
> J
>
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