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Comment by fulafel | Hacker News Reader

I think this is typical behaviour with ext4 on Linux, if the application doesn't do fsync/fdatasync to flush the data to disk.

Depending on mount options, ext4fs does metadata journaling ensuring the FS itself is not borked, but not data journaling which would safeguard the file contents in event of unclean shutdown with pending writes in the caches.

The same phenomenon is at play when people complain that their log files contain NUL bytes after a crash. The file system metadata has been updated for the size of the file to fit the appended write, but the data itself was not written out yet.

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The current default is data=ordered, which should prevent this problem if the hardware doesn't lie. The data doesn't go in the journal, but it has to be written before the journal is committed.

There was a point where ext3 defaulted to data=writeback, which can definitely give you files full of null bytes.

And data=journal exists but is overkill for this situation.

It's likely because of delayed allocations (delalloc): https://issuetracker.google.com/issues/172227346#comment6

because the only guarantee which data=ordered provides is the security guarantee that stale data won't be revealed.

Yes, it's bad and breaks prefix append consistency, and does not match the documentation...

For more context, that's a comment from one of the ext4 main authors, Ted Ts'o. the other subsequent comment from him spells out the case more but sadly no spelled out NUL byte origin story I spotted from skimming.

The original report [0] shows the corruption due to NUL bytes at the end of the file (see the hexdump). This comment [1] from Ted Ts'o details the exact chain of events leading to it.

[0] https://issuetracker.google.com/issues/172227346 [1] https://issuetracker.google.com/issues/172227346#comment8

> which should prevent this problem if the hardware doesn't lie.

Or, one can take the ZFS approach and assume the hardware often lies :)

I do not know how zfs will overcome hardware lying. If its going to fetch data that is in the drives cache, how will it overcome the persistence problem ?

It will at the very least notice that the read data does not match the stored checksum and not return the garbage data to the application. In redundant (raidz) setups it will then read the data from another disk, and update the faulty disk. In a non-redundant setup (or if enough disks are corrupted) it will signal an IO error.

An error is preferred to silently returning garbage data!

The "zeroed-out file" problem is not about firmware lying though, it is about applications using fsync() wrongly or not at all. Look up the O_PONIES controversy.

Sure, due to their COW nature zfs and btrfs provide better behavior despite broken applications. But you can't solve persistence in the face of lying firmware.

Even thought zfs has some enhancements to not corrupt itself on such drives, if you run for example a database on top, all guarantees around commit go out the window.

"Renaming a file should always happen-after pending writes to that file" is not a big pony. I think it's a reasonable request even in the absence of fsync.

Well, for one rename() is not always meant to be durable. It can also be used for IPC, for example some mail servers use it to move mails between queues. Flushing before every rename is unexpected in that situation.

Fun fact: rename() is atomic with respect to running applications per POSIX, that the on-disk rename is also atomic is only incidental.

I'm not suggesting flushing for rename. If a file write and a rename happen shortly before power loss, and neither goes through, that's fine.

With this rule, three outcomes are acceptable: both occur, or neither occur, or just the file write happens. The unacceptable outcome is that just the rename happens.

("file write" here could mean a single write, or an open-write-close sequence, it doesn't particularly matter and I don't want to dig through old discussions in too much detail)

As an aside, can you still get the bad checksum file contents with zfs? Eg if it's a big database with its own checksums you might want to run a db level recovery on it.

It can’t tell if the drive is lying to it.

This was my theory too, its not going to help in -these- situations. I can't see how.

Actual file data ends up in the same transaction group (txg) as metadata if both are changed within the same txg commit (either flushed explicitly, due to recordsize/buffer limit being reached, or txg commit timeout - 5 seconds by default), so if there is a write barrier violation caused by hardware lies followed by an untimely loss of power, the checksums for the txg updates won't match and they get rolled-back until the last valid one when importing the pool - which doesn't end up zero'ing out extents of a file (like in xfs) or ending up with a zero file size (like on ext3/ext4).

The "data" setting of ext filesystems isn't replacement for fsync().

It's not a replacement but it can give you some guarantees.

Also fsync is a terrible API that should be replaced, but that's mostly a different topic.

At least on linux you can use io_uring to make fsync asynchronous. And you can initiate some preparatory flushing with sync_file_range and only do the final commit with fsync to cut down the latency.

My only n=1 observation is that null values in logs occur on nvme, ssd and spinning rust. All ext4 with defaults. I do have the idea it occurs more on nvme drives though. But maybe my systems settings are just booked.

I don't think that's how it works: Flushing metadata before data would be a security concern (consider e.g. the metadata change of increasing a file's length due to an append before the data change itself), so file systems usually only ever do the opposite, which is safe.

Getting back zeroes after a metadata sync (which must follow a data sync) would accordingly be an indication of something weird having happened at the disk level: We'd expect to either see no data at all, or correct data, but not zeroes or any other file's or previously written stale data.

The file isn't stored contiguously on disk, so that would depend on the implementation of the filesystem. Perhaps the size of the file can be changed, without extents necessarily being allocated to cover the new size?

I seem to vaguely recall an issue like that, for ext4 in particular. Of course it's possible in general for any filesystem that supports holes, but I don't think we can necessarily assume that the data is always written, and all the pointers to it also written, before the file-size gets updated.

At least for ext4 and actually written data (i.e. not ftruncate’d files), I believe zeroes should really not occur.

Both extents and the file size are metadata as far as I understand, which would be atomically updated through the journal.

Data can be written before metadata (in data=ordered mode):

> All data are forced directly out to the main file system prior to its metadata being committed to the journal.

Is data=ordered the default?

As far as I can tell it's always been on both ext3 and ext4, yes.

I think there could semi-reasonably be case for the zero bytes appearing if the fs knows there should be something written there, and the block is has been allocated, but no write yet. Then it's not compromising confidentiality to zero the allocated block when recovering the journal when the disk is mounted. But the zero byte origin doesn't seem to be spelled out anywhere so this is just off the cuff reasoning.

The file's size could have been set by the application before copying data to it. This will result in a file which reads all zeroes.

Or if it were a hardware ordering fault, remember that SSD TRIM is typically used by modern filesystems to reclaim unused space. TRIMmed blocks read as zero.

> The file's size could have been set by the application before copying data to it. This will result in a file which reads all zeroes.

Hm, is that a common approach? I thought applications mostly use fallocate(2) for that if it's for performance reasons, which does not change the nominal file size.

Actually allocating zeroes sounds like it could be quite inefficient and confusing, but then again, fallocate is not portable POSIX.

> Or if it were a hardware ordering fault

That's what I suspect might be going on here.

fseek to the (new) end and back. Avoids metadata updates on every write. Not sure how common it is but it does occur.

Ext3 will totally let you expose yourself to those security issues. I'm not sure about ext4.

Only in data=writeback mode, which is not the default in either ext3 or ext4.

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