Ext4 Advances As Interim Step To Btrfs

Heise.de's Kernel Log has a look at the ext4 filesystem as Linus Torvalds has integrated a large collection of patches for it into the kernel main branch. "This signals that with the next kernel version 2.6.28, the successor to ext3 will finally leave behind its 'hot' development phase." The article notes that ext4 developer Theodore Ts'o (tytso) is in favor of ultimately moving Linux to a modern, "next-generation" file system. His preferred choice is btrfs, and Heise notes an email Ts'o sent to the Linux Kernel Mailing List a week back positioning ext4 as a bridge to btrfs.

Comments

Why not ZFS?

[mlts]

Unless ZFS has patent issues, why not just work on having ZFS as Linux's standard FS, after ext3?

ZFS offers a lot of capabilities, from no need to worry about a LVM layer, to snapshotting, to excellent error detection, even encryption and compression hooks.

Re:Why not ZFS?

[GrievousMistake]

Huh. One of the interesting things things about Reiser4 from an end-user perspective was Hans Reisers plans for file metadata. From what I can find about btrfs, it currently doesn't even support normal extended attributes. There was also talk about making it easy for developers to extend the filesystem with plugins that could add e.g. compression schemes.
I can't really recognize anything from Hans Reiser's ramblings in the btrfs documentation that isn't standard file system improvements already seen in e.g. ZFS. does anyone have any specific examples of the ZFS-leapfrogging features referred to?

Re:Why not ZFS?

[adrianwn]

A microkernel loads modules into the kernel space.

No, that's the opposite of a microkernel. A microkernel loads its modules (then often called "servers") into user space. If the kernel and its drivers etc. run in the same address space (as is the case with, e.g., Linux), then we're talking about a monolithic kernel, even if it can dynamically load modules.

You're both right.

[SanityInAnarchy]

ZFS duplicates a lot of functionality that belongs outside of a filesystem.

Very true.

It wouldn't be possible to duplicate RAID-Z with LVM.

Also true.

And the features which could be duplicated, couldn't be done nearly as well without a little more knowledge of the filesystem.

The real problem here is that we're finding out that generic block devices aren't enough to do everything we want to do outside the filesystem itself. Or, if they are, it's incredibly clumsy. Trivial example: If I want a copy-on-write snapshot, I have to set aside (ahead of time) some fixed amount of space that it can expand into. If I guess high, I waste space. If I guess low, I have to either expand it (somehow, if that's even possible) or lose my snapshot.

A filesystem which natively implemented COW could also trivially implement snapshots which take up exactly as much space as there are differences between the increments. But because of the way the Linux VFS is structured, this kind of functionality would have to be in a single filesystem, and would be duplicated across all filesystems. Best case, it'd be like ext3's JBD, as a kind of shared library.

A humble proposal: We need another layer, between the block layer and the filesystem layer -- call it an extent layer -- which is simply concerned with allocating some amount of space, and (perhaps) assigning it a unique ID. Filesystems could sit above this layer and implement whatever crazy optimizations or semantics they want -- linear vs btree vs whatever for directories, POSIX vs SQL, whatever.

The extent layer itself would only be concerned with allocating extents of some requested size, and actually storing the data. But this would be enough information to effectively handle mirroring, striping, snapshotting, copy-on-write, etc.

It wouldn't be universal -- I've said nothing about the on-disk format, and, indeed, some filesystems exist on Linux solely for that purpose -- vfat, ntfs, udf, etc. Those filesystems could be done pretty much exactly the way they're done now. After all, the existence of a block layer in no way implies that every filesystem must be tied to a block device (see proc, sys, fuse, etc.)

But I think it would work very well for filesystems which did choose to implement it. I think it would provide the best of ZFS and LVM.

I haven't actually been seriously following filesystem development for years, so maybe this is already done. Or maybe it's a bad idea. If not, hopefully some kernel developers are reading this.

Re:when ext4 is feature complete it will be the #3

[Jah-Wren Ryel]

The weakness with linux is in the LVM or EVMS layer. They both suck in that they are not enterprise ready (ie multi TB filesystems, 100+ MB/s sustained read/write) in that they cause unexplained IO hicups, lockups and kernel panics. LVM/EVMS certainly work fine for Joe Blow's HTPC, or a paltry 100GB database but they fall down when under serious load.

LVM has been rock-solid for me with a ~7TB and 2 2TB ext3 filesystems (24 500GB disks) over the course of a year and a half. No problems migrating extents all over the place when I needed to swap disks in and out. Almost identical to HPUX in functionality, but without the sizing constraints.

But, when I tried xfs for kicks I found out that a 7TB filesystem means you need 7GB of RAM to fsck it - impossible on a 32-bit system, I also had a week where I it all went in the shitter because I ran free-space to zero and started getting OS panics and data corruption.

I'm definitely considering jfs for the next generation, my main complaint with ext3 has been ridiculously slow deletes and fsck's. Problems I have read don't exist with jfs.