Re: [Qemu-devel] Re: [PATCH] virtio-spec: document block CMD and FLUSH
Rusty Russell wrote: On Wed, 5 May 2010 05:47:05 am Jamie Lokier wrote: Jens Axboe wrote: On Tue, May 04 2010, Rusty Russell wrote: ISTR someone mentioning a desire for such an API years ago, so CC'ing the usual I/O suspects... It would be nice to have a more fuller API for this, but the reality is that only the flush approach is really workable. Even just strict ordering of requests could only be supported on SCSI, and even there the kernel still lacks proper guarantees on error handling to prevent reordering there. There's a few I/O scheduling differences that might be useful: 1. The I/O scheduler could freely move WRITEs before a FLUSH but not before a BARRIER. That might be useful for time-critical WRITEs, and those issued by high I/O priority. This is only because noone actually wants flushes or barriers, though I/O people seem to only offer that. We really want these writes must occur before this write. That offers maximum choice to the I/O subsystem and potentially to smart (virtual?) disks. We do want flushes for the D in ACID - such things as after receiving a mail, or blog update into a database file (could be TDB), and confirming that to the sender, to have high confidence that the update won't disappear on system crash or power failure. Less obviously, it's also needed for the C in ACID when more than one file is involved. C is about differently updated things staying consistent with each other. For example, imagine you have a TDB file mapping Samba usernames to passwords, and another mapping Samba usernames to local usernames. (I don't know if you do this; it's just an illustration). To rename a Samba user involves updating both. Let's ignore transient transactional issues :-) and just think about what happens with per-file barriers and no sync, when a crash happens long after the updates, and before the system has written out all data and issued low level cache flushes. After restarting, due to lack of sync, the Samba username could be present in one file and not the other. 2. The I/O scheduler could move WRITEs after a FLUSH if the FLUSH is only for data belonging to a particular file (e.g. fdatasync with no file size change, even on btrfs if O_DIRECT was used for the writes being committed). That would entail tagging FLUSHes and WRITEs with a fs-specific identifier (such as inode number), opaque to the scheduler which only checks equality. This is closer. In userspace I'd be happy with a all prior writes to this struct file before all future writes. Even if the original guarantees were stronger (ie. inode basis). We currently implement transactions using 4 fsync /msync pairs. write_recovery_data(fd); fsync(fd); msync(mmap); write_recovery_header(fd); fsync(fd); msync(mmap); overwrite_with_new_data(fd); fsync(fd); msync(mmap); remove_recovery_header(fd); fsync(fd); msync(mmap); Yet we really only need ordering, not guarantees about it actually hitting disk before returning. In other words, FLUSH can be more relaxed than BARRIER inside the kernel. It's ironic that we think of fsync as stronger than fbarrier outside the kernel :-) It's an implementation detail; barrier has less flexibility because it has less information about what is required. I'm saying I want to give you as much information as I can, even if you don't use it yet. I agree, and I've started a few threads about it over the last couple of years. An fsync_range() system call would be very easy to use and (most importantly) easy to understand. With optional flags to weaken it (into fdatasync, barrier without sync, sync without barrier, one-sided barrier, no lowlevel cache-flush, don't rush, etc.), it would be very versatile, and still easy to understand. With an AIO version, and another flag meaning don't rush, just return when satisfied, and I suspect it would be useful for the most demanding I/O apps. -- Jamie ___ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linux-foundation.org/mailman/listinfo/virtualization
Re: [Qemu-devel] Re: [PATCH] virtio-spec: document block CMD and FLUSH
On Wed, 5 May 2010 14:28:41 +0930 Rusty Russell ru...@rustcorp.com.au wrote: On Wed, 5 May 2010 05:47:05 am Jamie Lokier wrote: Jens Axboe wrote: On Tue, May 04 2010, Rusty Russell wrote: ISTR someone mentioning a desire for such an API years ago, so CC'ing the usual I/O suspects... It would be nice to have a more fuller API for this, but the reality is that only the flush approach is really workable. Even just strict ordering of requests could only be supported on SCSI, and even there the kernel still lacks proper guarantees on error handling to prevent reordering there. There's a few I/O scheduling differences that might be useful: 1. The I/O scheduler could freely move WRITEs before a FLUSH but not before a BARRIER. That might be useful for time-critical WRITEs, and those issued by high I/O priority. This is only because noone actually wants flushes or barriers, though I/O people seem to only offer that. We really want these writes must occur before this write. That offers maximum choice to the I/O subsystem and potentially to smart (virtual?) disks. 2. The I/O scheduler could move WRITEs after a FLUSH if the FLUSH is only for data belonging to a particular file (e.g. fdatasync with no file size change, even on btrfs if O_DIRECT was used for the writes being committed). That would entail tagging FLUSHes and WRITEs with a fs-specific identifier (such as inode number), opaque to the scheduler which only checks equality. This is closer. In userspace I'd be happy with a all prior writes to this struct file before all future writes. Even if the original guarantees were stronger (ie. inode basis). We currently implement transactions using 4 fsync /msync pairs. write_recovery_data(fd); fsync(fd); msync(mmap); write_recovery_header(fd); fsync(fd); msync(mmap); overwrite_with_new_data(fd); fsync(fd); msync(mmap); remove_recovery_header(fd); fsync(fd); msync(mmap); Seems over-zealous. If the recovery_header held a strong checksum of the recovery_data you would not need the first fsync, and as long as you have two places to write recovery data, you don't need the 3rd and 4th syncs. Just: write_internally_checksummed_recovery_data_and_header_to_unused_log_space() fsync / msync overwrite_with_new_data() To recovery you choose the most recent log_space and replay the content. That may be a redundant operation, but that is no loss. Also cannot see the point of msync if you have already performed an fsync, and if there is a point, I would expect you to call msync before fsync... Maybe there is some subtlety there that I am not aware of. Yet we really only need ordering, not guarantees about it actually hitting disk before returning. In other words, FLUSH can be more relaxed than BARRIER inside the kernel. It's ironic that we think of fsync as stronger than fbarrier outside the kernel :-) It's an implementation detail; barrier has less flexibility because it has less information about what is required. I'm saying I want to give you as much information as I can, even if you don't use it yet. Only we know that approach doesn't work. People will learn that they don't need to give the extra information to still achieve the same result - just like they did with ext3 and fsync. Then when we improve the implementation to only provide the guarantees that you asked for, people will complain that they are getting empty files that they didn't expect. The abstraction I would like to see is a simple 'barrier' that contains no data and has a filesystem-wide effect. If a filesystem wanted a 'full' barrier such as the current BIO_RW_BARRER, it would send an empty barrier, then the data, then another empty barrier. (However I suspect most filesystems don't really need barriers on both sides.) A low level driver might merge these together if the underlying hardware supported that combined operation (which I believe some do). I think this merging would be less complex that the current need to split a BIO_RW_BARRIER in to the three separate operations when only a flush is possible (I know it would make md code a lot nicer :-). I would probably expose this to user-space as extra flags to sync_file_range: SYNC_FILE_RANGE_BARRIER_BEFORE SYNC_FILE_RANGE_BARRIER_AFTER This would make it clear that a barrier does *not* imply a sync, it only applies to data for which a sync has already been requested. So data that has already been 'synced' is stored strictly before data which has not yet been submitted with write() (or by changing a mmapped area). The barrier would still be filesystem wide in that if you SYNC_FILE_WRITE_WRITE one file, then SYNC_FILE_RANGE_BARRIER_BEFORE another file on the same filesystem, the pages scheduled in the first file would be affect by the barrier request on the second file. Implementing
Re: [Qemu-devel] Re: [PATCH] virtio-spec: document block CMD and FLUSH
On Wed, 5 May 2010 05:47:05 am Jamie Lokier wrote: Jens Axboe wrote: On Tue, May 04 2010, Rusty Russell wrote: ISTR someone mentioning a desire for such an API years ago, so CC'ing the usual I/O suspects... It would be nice to have a more fuller API for this, but the reality is that only the flush approach is really workable. Even just strict ordering of requests could only be supported on SCSI, and even there the kernel still lacks proper guarantees on error handling to prevent reordering there. There's a few I/O scheduling differences that might be useful: 1. The I/O scheduler could freely move WRITEs before a FLUSH but not before a BARRIER. That might be useful for time-critical WRITEs, and those issued by high I/O priority. This is only because noone actually wants flushes or barriers, though I/O people seem to only offer that. We really want these writes must occur before this write. That offers maximum choice to the I/O subsystem and potentially to smart (virtual?) disks. 2. The I/O scheduler could move WRITEs after a FLUSH if the FLUSH is only for data belonging to a particular file (e.g. fdatasync with no file size change, even on btrfs if O_DIRECT was used for the writes being committed). That would entail tagging FLUSHes and WRITEs with a fs-specific identifier (such as inode number), opaque to the scheduler which only checks equality. This is closer. In userspace I'd be happy with a all prior writes to this struct file before all future writes. Even if the original guarantees were stronger (ie. inode basis). We currently implement transactions using 4 fsync /msync pairs. write_recovery_data(fd); fsync(fd); msync(mmap); write_recovery_header(fd); fsync(fd); msync(mmap); overwrite_with_new_data(fd); fsync(fd); msync(mmap); remove_recovery_header(fd); fsync(fd); msync(mmap); Yet we really only need ordering, not guarantees about it actually hitting disk before returning. In other words, FLUSH can be more relaxed than BARRIER inside the kernel. It's ironic that we think of fsync as stronger than fbarrier outside the kernel :-) It's an implementation detail; barrier has less flexibility because it has less information about what is required. I'm saying I want to give you as much information as I can, even if you don't use it yet. Thanks, Rusty. ___ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linux-foundation.org/mailman/listinfo/virtualization
