From: Shawn O. Pearce Date: Thu, 05 Apr 2007 15:53:06 GMT Subject: Re: [PATCH] git-{repack,pack-objects} accept --{pack,blob}-limit to control pack size Message-ID: <20070405155306.GI5436@spearce.org> In-Reply-To: Linus Torvalds wrote: > > On Thu, 5 Apr 2007, Shawn O. Pearce wrote: > > > > For glibc we do try to set _FILE_OFFSET_BITS to 64 > > I repeat: that's _broken_. > > It's in no way portable. It's a glibc horror. It should not be used. > > It was a quick hack, but the real way to do it is to use "loff_t" and > "llseek". Sure, OK, but that libc function doesn't exist on Mac OS X: man llseek: This function is Linux-specific, and should not be used in programs intended to be portable. So we'd need our own horror to wrap llseek as an lseek fake-alike anyway. That's what that glibc horror does, and we didn't have to write that code. :-) > But there simply isn't any way to do mmap() or pread() portably outside > the 32-bit area. So there are good reasons why we should just limit > pack-files to 32-bits on 32-bit architectures. Not unless your off_t is 64 bits, no. If it is 64 bits then you should be able to do a pread or mmap starting past the first 2 GiB. You just might not be able to ask for a mmap that exceeds 2 GiB in size, as your size_t may not be that large. E.g., Darwin/Mac OS X. Hence the sliding window mmap. > So I think that Dana's approach is just fundamentally correct. Yeah, we > should probably have a 64-bit index as a *possibility*, but it simply > isn't a replacement for "keep packs under 2GB in size". I'm not disagreeing. Some filesystems (FAT on a USB stick, Dana's example) just don't want files larger than 2 GiB. So keeping them small has a number of advantages. Plus they are easier to burn on DVD: 2 packs per DVD. ;-) I was simply trying to point out that the mmap code isn't broken if the off_t is able to handle a file of that size; and if it can't then other things are broken, like a simple lseek. -- Shawn.