On Thu, Sep 04, 2014 at 04:10:14PM +0200, Kevin Wolf wrote: > Am 04.09.2014 um 15:51 hat Stefan Hajnoczi geschrieben: > > On Thu, Sep 04, 2014 at 06:07:32AM -0600, Eric Blake wrote: > > > On 09/04/2014 02:58 AM, Stefan Hajnoczi wrote: > > > > On-disk structures should be marked packed so the compiler does not > > > > insert padding for field alignment. Padding should be explicit so > > > > on-disk layout is obvious and we don't rely on the architecture-specific > > > > ABI for alignment rules. > > > > > > > > The pahole(1) diff shows that the padding is now explicit and offsets > > > > are unchanged: > > > > > > > > char backing_file[1024]; /* 8 1024 > > > > */ > > > > /* --- cacheline 16 boundary (1024 bytes) was 8 bytes ago --- */ > > > > int32_t mtime; /* 1032 4 > > > > */ > > > > - > > > > - /* XXX 4 bytes hole, try to pack */ > > > > - > > > > + uint32_t padding; /* 1036 4 > > > > */ > > > > uint64_t size; /* 1040 8 > > > > */ > > > > > > Was a 32-bit build also inserting this padding, or do we have historical > > > differences where 32-bit and 64-bit cow files are actually different, > > > and we may need to be prepared to parse files from both sources? > > > > Good point. Let's not merge this patch since it breaks 32-bit hosts. > > > > The fact that no one hit problems when exchanging files between 32-bit > > and 64-bit machines shows that the cow format is rarely used. > > > > At this point we have 2 different formats: one without padding > > (i386-style) and one with padding (x86_64-style). The chance of more > > variants is small but who knows, maybe some other host architecture ABI > > has yet another alignment rule for uint64_t. > > > > I'd like to git rm block/cow.c but I suppose the backwards-compatible > > thing to do is to introduce subformats to support both variants. > > Opinions? > > Can we safely detect which of the subformats we have? But I'm not sure > if it's even worth fixing.
I think it would default to the subformat depending on the host architecture but allow overriding with -o subformat=i386|x86_64. I'm also not sure if it's worth fixing. The cow file format is so rarely used I wonder if we'd be better off without it. Stefan
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