[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Qemu-block] [Qemu-devel] [PATCH 2/4] nbd/server: Advertise actual m

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH 2/4] nbd/server: Advertise actual minimum block size
Date: Fri, 17 Aug 2018 18:04:26 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0

17.08.2018 17:53, Eric Blake wrote:
On 08/17/2018 08:49 AM, Vladimir Sementsov-Ogievskiy wrote:

+    /* minimum - 1 for back-compat, or actual if client will obey it. */
+    if (client->opt == NBD_OPT_INFO || blocksize) {
+        sizes[0] = blk_get_request_alignment(exp->blk);
+    } else {
+        sizes[0] = 1;
+    }
      /* preferred - Hard-code to 4096 for now.
       * TODO: is blk_bs(blk)->bl.opt_transfer appropriate? */
      sizes[1] = 4096;

Here, what about dirty bitmap export through BLOCK_STATUS? Could it's granularity be less than request_alignment? Shouldn't we handle it somehow?

Can you create a dirty bitmap with a granularity smaller than request_alignment?  I know you can configure dirty bitmap granularity independently from cluster size (in both directions: either smaller or larger than cluster size), but that it has a least a minimum lower bounds of 512.  You're probably right that we also want it to have a minimum lower bound of the request_alignment (if you're using a device with 4k minimum I/O, request_alignment would be 4k, and having a dirty bitmap any smaller than that granularity is wasted space).

On the other hand, I also think we're safe for this patch: even if you waste the space by creating a bitmap with too-small granularity, the actions that write bits into the bitmap will still be aligned to request_alignment,

Not quite right: nbd_export_bitmap searches through the backing chain for the bitmap, and it may correspond to the bds with smaller request_alignment.

which means you always set a multiple of bits per action; when reading back the dirty bitmap to report over NBD_CMD_BLOCK_STATUS, you'll never encounter a change in bit status except on alignment boundaries (based on how the bits were written), and thus what NBD reports will still be aligned to the advertised minimum size, rather than the smaller bitmap granularity.

Best regards,

reply via email to

[Prev in Thread] Current Thread [Next in Thread]