On 2013-09-18 22:52, Sage Weil wrote:
Hmm, looking at the code, I'm surprised that this isn't working. The C
aio_read call is just doing
bufferlist bl;
bufferptr bp = buffer::create_static(len, buf);
bl.push_back(bp);
ret = ctx->read(oid, bl, len, off);
if (ret >= 0) {
if (bl.length() > len)
return -ERANGE;
if (bl.c_str() != buf)
bl.copy(0, bl.length(), buf);
Hey Sage,
thanks for the hints. You're citing the synchronous version of
rados_read, not rados_aio_read. The difference between rados_read and
rados_aio_read (the C-versions) is that rados_read uses a bufferlist,
and rados_aio_read uses an overload with a char* buf. Both are delegated
to IoCtxImpl. IoCtxImpl has two overloads for aio_read, one accepting a
bufferlist and a char*, but only one overload for read, accepting a
bufferlist only.
IoCtxImpl's overloads for aio_read are almost identical, the difference
is that the buflist overload sets a bufferlist on AioCompletionImpl* c,
c->pbl = pbl;
and the char* buffer-overload sets a buffer
c->buf = buf;
AioCompletionImpl contains multiple data members: a bufferlist (bl), a
pointer to a bufferlist (pbl), and a pointer to a character array buf.
The following calls (in IoCtxImpl's aio_read overloads) to the objecter
are identical:
objecter->read(oid, oloc,
off, len, snapid, &c->bl, 0,
onack, &c->objver);
Looking at the Objecter read and deeper in the call chain, it seems that
information about data member pbl in AioCompletionImpl* is lost. The
objecter only knows about a Context, not about AioCompletionImpl. The
user-provided buffer is not passed on.
My preliminary conclusion is that my problem is caused by information
lost in IoCtxImpl's aio_read overloads. Maybe it can be solved by
modifying IoCtxImpl.cc:
* removing line 615. Not sure why the AioCompletionImpl needs to know
anything about buffers?
* replacing '&c->bl' with 'pbl' on line 619 of IoCtxImpl.cc, making
the call to the objecter
objecter->read(oid, oloc,
off, len, snapid, pbl, 0,
onack, &c->objver);
In that way, the bufferlist is passed through, and not thrown away.
Thanks,
Rutger
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