Messages in this thread | | | Date | Mon, 25 Feb 2013 16:49:54 -0500 | From | Ric Wheeler <> | Subject | Re: New copyfile system call - discuss before LSF? |
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On 02/25/2013 04:14 PM, Andy Lutomirski wrote: > On 02/21/2013 02:24 PM, Zach Brown wrote: >> On Thu, Feb 21, 2013 at 08:50:27PM +0000, Myklebust, Trond wrote: >>> On Thu, 2013-02-21 at 21:00 +0100, Paolo Bonzini wrote: >>>> Il 21/02/2013 15:57, Ric Wheeler ha scritto: >>>>>> sendfile64() pretty much already has the right arguments for a >>>>>> "copyfile", however it would be nice to add a 'flags' parameter: the >>>>>> NFSv4.2 version would use that to specify whether or not to copy file >>>>>> metadata. >>>>> That would seem to be enough to me and has the advantage that it is an >>>>> relatively obvious extension to something that is at least not totally >>>>> unknown to developers. >>>>> >>>>> Do we need more than that for non-NFS paths I wonder? What does reflink >>>>> need or the SCSI mechanism? >>>> For virt we would like to be able to specify arbitrary block ranges. >>>> Copying an entire file helps some copy operations like storage >>>> migration. However, it is not enough to convert the guest's offloaded >>>> copies to host-side offloaded copies. >>> So how would a system call based on sendfile64() plus my flag parameter >>> prevent an underlying implementation from meeting your criterion? >> If I'm guessing correctly, sendfile64()+flags would be annoying because >> it's missing an out_fd_offset. The host will want to offload the >> guest's copies by calling sendfile on block ranges of a guest disk image >> file that correspond to the mappings of the in and out files in the >> guest. >> >> You could make it work with some locking and out_fd seeking to set the >> write offset before calling sendfile64()+flags, but ugh. >> >> ssize_t sendfile(int out_fd, int in_fd, off_t in_offset, off_t >> out_offset, size_t count, int flags); >> >> That seems closer. >> >> We might also want to pre-emptively offer iovs instead of offsets, >> because that's the very first thing that's going to be requested after >> people prototype having to iterate calling sendfile() for each >> contiguous copy region. > I thought the first thing people would ask for is to atomically create a > new file and copy the old file into it (at least on local file systems). > The idea is that nothing should see an empty destination file, either > by race or by crash. (This feature would perhaps be described as a > pony, but it should be implementable.) > > This would be like a better link(2). > > --Andy
Why would this need to be atomic? That would seem to be a very difficult property to provide across all target types with multi-GB sized files...
Ric
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