2012-08-25 15:46, Sašo Kiselkov wrote:
The difference is that when you want to go fetch a block from a metaxel, you still need some way to reference it. Either you use direct references (i.e. ARC entries as above), or you use an indirect mechanism, which means that for each read you will need to walk the metaxel device, which is slow.
Um... how does your application (or the filesystem during its metadata traversal) know that it wants to read a certain block? IMHO, it has the block's address for that (likely known from a higher-level "parent" block of metadata), and it requests - "give me L bytes from offset O on tlvdev T", which is the layman interpretation of DVA. From what I understand, with ARC-cached blocks, we traverse the RAM-based cache and find one with the requested DVA; then we have its data already in RAM and return it to the caller. If the block is not in ARC (and there's no L2ARC), we can fetch it from the media using the DVA address(es?) we already know from the request. In case of L2ARC there is probably a non-null pointer to the l2arc_buf_hdr_t, so we can request the block from the L2ARC. If true, this is not faster than fetching the block from the same SSD used as a metadata accelerator instead of being an L2ARC device with a policy (or even without one, as today), and in comparison only wasted RAM for the ARC entries. BTW, as I see in "struct arc_buf_hdr", it only stores one DVA - so I guess for blocks with multiple on-disk copies it is possible to have them cached twice, or does ZFS always enforce storing and seeking the ARC by a particular DVA of the block (likely DVA[0])? http://src.illumos.org/source/xref/illumos-gate/usr/src/uts/common/fs/zfs/arc.c#433 If we do go with METAXELs as I described/proposed, and prefer fetching metadata from SSD unless there are errors, then some care should be taken to use this instance of DVA to reference cached metadata blocks in the ARC. //Jim Klimov _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
