[lustre-devel] Request arc buffer, zerocopy

Anna Fuchs anna.fuchs at informatik.uni-hamburg.de
Fri Jun 28 02:50:12 PDT 2019

Hello Matt,

thanks for your reply.

> You can set the block size (of the first and only block) using 
> dmu_object_set_blocksize().  FYI, I think that this comment is 
> incorrect:
>  * If the first block is allocated already, the new size must be 
> greater
>  * than the current block size.
> You can increase or decrease the block size with this routine.

This is a deeper call of Lustre's osd_grow_blocksize I mentioned 
before. If I understand it correctly, they are called in the context of 
transactions, right?
If so, I can not use it - I need the blocksize to be set in the buffer 
preparation stage, before comitting in a transaction.
Lustre's original routine looks simplified as follows:

	bs = dn->dn_datablksz
	while (len > 0)
		if (sz_in_block == bs) 		/* full block, try zerocopy */
			abuf = osd_request_arcbuf(dn, bs);
		else 				/* can't use zerocopy, allocate temp. buffers */
			... alloc_page ...
			here going later on the dmu_write path (pagewise!)

Currently, in the very first iteration this blocksize (bs) is taken 
from the dnode and is e.g. 4K.
When writing a chunk of 16K, I get 4 arcbufs 4K each. For the next 
chunk the block size might be grown up to x (recordsize?).
Here I need the blocksize to be set to 16K (or 128K or later some 
generic value defined by the Lustre client) before the first arcbuf is 
because the compressed chunk sent from client is logically this size.
At this point I don't have any dmu_tx yet to grow the blocksize saved 
in dn->dn_datablksz before the while loop.
So I am not sure how deep to go? This min size is set on dnode creation 
by ZFS, how can I "reset" it?

> I'd recommend that you hand the compressed data to ZFS similarly to 
> how "zfs receive" does (for compressed send streams).  It sounds like 
> the is the direction you're going, which is great.  FYI, here are 
> some of the routines you'd want to use (copied from dmu_recv.c):
> 			abuf = arc_loan_compressed_buf(
> 			   dmu_objset_spa(drc->drc_os),
> 			   drrw->drr_compressed_size, drrw->drr_logical_size,
> 			   drrw->drr_compressiontype);
> 	dmu_assign_arcbuf(bonus, drrw->drr_offset, abuf, tx);
>  (or dmu_assign_arcbuf_dnode())
> 			dmu_return_arcbuf(rrd->write_buf);

Yes, thanks for that. We have two paths how Lustre interacts with ZFS - 
requesting arc buffers or dmu_write.
The common dmu_request_arcbuf goes over arc_loan_buf, so we introduced 
dmu_request_compressed_arcbuf to go over arc_loan_compressed_buf to 
reuse the receive functionality.
We try to make as few changes as possible on Lustre's interface since 
we want mix compressed and uncompressed data chunks (and be at the same 
time compatible with ZFS' on disk format..)
The dmu_write path will be tricky, though.

Any comments are welcome.

Best regards


Anna Fuchs
Universität Hamburg

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