[Sheepdog] [PATCH] sheep: reduce snapshot COW read/write

[HaiTing Yao]

On Thu, Mar 22, 2012 at 5:15 PM, Liu Yuan <namei.unix at gmail.com> wrote:

>  On 03/22/2012 04:40 PM, yaohaiting.wujue at gmail.com wrote:
>
> > From: HaiTing Yao <wujue.yht at taobao.com>
> >
> > Doing snapshot COW:
> >
> > 1, If new writing request occurs and need COW for sanpshot, now read old
> > object to buffer, then write the buffer to new object, then write the
> > request data to new object. We can merge the latter two writing request.
> >
> > 2, If new writing request covers whole object, no need to read old
> > object.
> >
> > After the modification, pass bigger buffer to do_write_obj when doing
> > COW, but it will not add the burden. COW is never for inode object, so
> > it will not use the journal.
> >
> > Signed-off-by: HaiTing Yao <wujue.yht at taobao.com>
> > ---
> >  sheep/store.c |   29 +++++++++++++++++------------
> >  1 files changed, 17 insertions(+), 12 deletions(-)
> >
> > diff --git a/sheep/store.c b/sheep/store.c
> > index dfec235..97f1f13 100644
> > --- a/sheep/store.c
> > +++ b/sheep/store.c
> > @@ -640,6 +640,7 @@ int store_create_and_write_obj(const struct sd_req
> *req, struct sd_rsp *rsp, voi
> >  {
> >       struct sd_obj_req *hdr = (struct sd_obj_req *)req;
> >       struct request *request = (struct request *)data;
> > +     struct sd_obj_req cow_hdr;
> >       int ret;
> >       uint32_t epoch = hdr->epoch;
> >       char *buf = NULL;
> > @@ -664,19 +665,23 @@ int store_create_and_write_obj(const struct sd_req
> *req, struct sd_rsp *rsp, voi
> >               dprintf("%" PRIu64 ", %" PRIx64 "\n", hdr->oid,
> hdr->cow_oid);
> >
> >               buf = xzalloc(SD_DATA_OBJ_SIZE);
> > -             ret = read_copy_from_cluster(request, hdr->epoch,
> hdr->cow_oid, buf);
> > -             if (ret != SD_RES_SUCCESS) {
> > -                     eprintf("failed to read cow object\n");
> > -                     goto out;
> > +             if ((hdr->data_length != SD_DATA_OBJ_SIZE)
> > +                     || (hdr->offset!= 0)) {
>
>
> For create_and_write object, data_length of request is always
> SD_DATA_OBJ_SIZE and offset always 0.
>
> Thanks,
> Yuan
>

Yes, usually the offset is 0 and size is SD_DATA_OBJ_SIZE , but there is
one exception.

for example:
1) write one vdi with ID of 0x19128 from 0 to  SD_DATA_OBJ_SIZE(write the
first whole object)
2)create snapshot for this vdi, get one new vdi with ID 0x19129
3)write vdi again, from 1024 to 2048 this time
4)need read data from 0x19128-00000000, then write data to
0x19129-00000000, so occurs COW
5)the hdr offset is 1024, and size is 1024 now

Thanks
Haiting

>
> > +                     ret = read_copy_from_cluster(request, hdr->epoch,
> hdr->cow_oid, buf);
> > +                     if (ret != SD_RES_SUCCESS) {
> > +                             eprintf("failed to read cow object\n");
> > +                             goto out;
> > +                     }
> >               }
> > -             iocb.buf = buf;
> > -             iocb.length = SD_DATA_OBJ_SIZE;
> > -             iocb.offset = 0;
> > -             ret = sd_store->write(hdr->oid, &iocb);
> > -             if (ret != SD_RES_SUCCESS)
> > -                     goto out;
> > -     }
> > -     ret = do_write_obj(&iocb, hdr, epoch, request->data);
> > +
> > +             memcpy(buf + hdr->offset, request->data, hdr->data_length);
> > +             memcpy(&cow_hdr, hdr, sizeof(cow_hdr));
> > +             cow_hdr.offset = 0;
> > +             cow_hdr.data_length = SD_DATA_OBJ_SIZE;
> > +
> > +             ret = do_write_obj(&iocb, &cow_hdr, epoch, buf);
> > +     } else
> > +             ret = do_write_obj(&iocb, hdr, epoch, request->data);
> >  out:
> >       free(buf);
> >       sd_store->close(hdr->oid, &iocb);
>
>
>
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