On Sat, 4 May 2013 13:33:26 -0400, Theodore Ts'o <tytso@xxxxxxx> wrote: > On Sat, May 04, 2013 at 10:58:50PM +0800, Ji Wu wrote: > > Hi, > > I have two questions regarding ext4_fallocate(), > > > > (1) The first is the FALLOC_FL_PUNCH_HOLE support, I am wondering > > what is the usage for it? The only use case comes to my mind is > > while ext4 being used for virtual machine image file storage. When > > VMM is aware of the file deleting operation in guest os, it can > > invoke host file system's fallocate() on the virtual machine image > > file to punch a hole to free host storage, so that save host > > space. But how can VMM being aware of guest file deleting? Simulate > > a virtual SSD-like block device to guest os, then capture the TRIM > > instruction issued by guest file system? That seems too tricky. So > > basically, where and how to benefit from hole punching? > > It's not too tricky; all of the hypervisors, whether it's KVM, or Xen, > or VMWare, are already simulating a SATA device to the guest OS. > Implementing support for the TRIM request is not that hard, and most > of the hypervisors are doing this already. Implementing the punch > hole functionality was indeed primarily motivated for this use case. > > The other historical use of this was for digital video recorders, but > that's a much more specialized use case. > > > (2) At the beginning of the function ext4_ext_punch_hole(), the > > codes are as follows, > > > > /* write out all dirty pages to avoid race condition */ > > filemap_write_and_wait_range(mapping, offset, offset+length-1); > > mutex_lock(&inode->i_mutex); > > truncate_page_cache_range(); > > > > Why does it need synchronously write back the dirty pages fit > > into the hole, the data on the disk responding to those pages are to > > be deleted, why not directly release those pages, no matter they are > > dirty or not. And furthermore, this is done before the inode lock is > > held, so it seems it may happen that after the pages are written > > back, and before the lock is held, those pages are dirtied again. > > So basically, why does it need call filemap_write_and_wait_range() > > before releasing those pages? > > That's a good a question. Looking at it, I'm not sure we do. I > suspect this was put in originally to avoid races with setting the > EOFBLOCKS_FL flag, but as you point out, there's no way we can prevent > writes to sneak in before we grab the i_mutex. As a result, we ended > up dropping the need for EOFBLOCKS_FL entirely. > > Maybe one of the ext4 developers will see something that I'm missing, > but I think we can drop this, which indeed will have a significant > performance improvement for systems that use the punch hole > functionality. Yes, there is a space for optimization here, but ordered case is special and we have to call analog of ext4_begin_ordered_truncate() with two arguments. > > Cheers, > > - Ted > -- > To unsubscribe from this list: send the line "unsubscribe linux-ext4" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
