Re: [Qemu-devel] [PATCH 2/2] hw/pci-host/x86: extend the 64-bit PCI hole relative to the fw-assigned base

[Laszlo Ersek]

On 09/27/18 17:24, Eric Blake wrote:
> On 9/26/18 3:35 PM, Laszlo Ersek wrote:
>> (+Eric)
> 
>> This too sounds useful. AIUI, ftruncate() is neither forbidden, nor
>> required, to allocate filesystem extents when increasing the size of a
>> file. Using one smaller regular temporary file as the common foundation
>> for multiple "memory-backend-file" objects will save space on the fs if
>> ftruncate() happens to allocate extents.
> 
> Correct. Most likely, ftruncate() extends by creating a hole if the file
> system supports sparse files, but POSIX has no way to guarantee holes
> (it merely states that both ftruncate() and write() beyond EOF are the
> two POSIX interfaces that may create holes - but that it is up to the
> implementation whether holes actually get created, then fstat() and
> lseek(SEEK_HOLE) can inform you whether holes were actually created [the
> former when st_blocks < st_size]). posix_fallocate() exists to force
> allocation, and Linux fallocate() offers even more fine-tuning when you
> need specific hole or non-hole behaviors.
> 

Thank you for the answers!
Laszlo