FAQ

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(Does Btrfs run with older kernels?)
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=== Does Btrfs run with older kernels? ===
 
=== Does Btrfs run with older kernels? ===
  
We maintain compatibility with kernels back to 2.6.18.  Kernels older than that will not work.
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v0.16 of Btrfs maintains compatibility with kernels back to 2.6.18.  Kernels older than that will not work.
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The current Btrfs unstable repositories only work against the mainline kernel.  Once Btrfs is in mainline a backport repository will be created again.
  
 
=== What is CRFS?  Is it related to BTRFS? ===
 
=== What is CRFS?  Is it related to BTRFS? ===

Revision as of 16:50, 11 December 2008

Contents

Related technologies and projects

How long will the Btrfs disk format keep changing?

We have an aggressive development schedule to get the disk format in shape for Btrfs 1.0. The plan is to finalize the disk format by the end of 2008. We might not have every single feature done that we want, but the disk format will be extensible enough for us to add those features later without requiring users to reformat.

Will conversion utilities be provided for disk format changes before 1.0?

We do understand that disk format changes make long term testing of Btrfs difficult, but it isn't practical for the small development team to maintain conversion tools along the way.

Will Btrfs be in the mainline Linux Kernel?

We will not submit Btrfs for inclusion to the mainline kernel until the disk format is finalized. After that, we'll work closely with the kernel community to get Btrfs in as quickly as possible.

Does Btrfs run with older kernels?

v0.16 of Btrfs maintains compatibility with kernels back to 2.6.18. Kernels older than that will not work.

The current Btrfs unstable repositories only work against the mainline kernel. Once Btrfs is in mainline a backport repository will be created again.

What is CRFS? Is it related to BTRFS?

CRFS is a network file system protocol. It was designed at around the same time as BTRFS. Its wire format uses some BTRFS disk formats and crfsd, a CRFS server implementation, uses BTRFS to store data on disk. More information can be found at http://oss.oracle.com/projects/crfs/

Does the Btrfs multi-device support make it a "rampant layering violation"?

Yes and no. Device management is a complex subject, and there are many different opinions about the best way to do it. Internally, the Btrfs code separates out components that deal with device management and maintains its own layers for them. The vast majority of filesystem metadata has no idea there are multiple devices involved.

Many advanced features such as checking alternate mirrors for good copies of a corrupted block are meant to be used with RAID implementations below the FS.

What checksum function does Btrfs use?

Currently Btrfs uses crc32c for data and metadata. The disk format has room for 256bits of checksum for metadata and up to a full leaf block (roughly 4k or more) for data blocks. Over time we'll add support for more checksum alternatives.

Can data checksumming be turned off?

Yes, you can disable it by mounting with -o nodatasum

Can copy-on-write be turned off for data blocks?

Yes, you can disable it by mounting with -o nodatacow. This implies -o nodatasum as well. COW may still happen if a snapshot is taken.

Is Btrfs optimized for SSD?

There are some optimizations for SSD drives, and you can enable them by mounting with -o ssd. Over the long term these will be enabled automatically. SSD is going to be a big part of future storage, and the Btrfs developers plan on tuning for it heavily.

Does Btrfs have data=ordered mode like Ext3?

Yes. In v0.16, Btrfs waits until data extents are on disk before updating metadata. This ensures that stale data isn't exposed after a crash, and that file data is consistent with the checksums stored in the btree after a crash.

Btrfs does not force all dirty data to disk on every fsync or O_SYNC operation.

Can the data=ordered mode be turned off in Btrfs?

No, it is an important part of keeping data and checksums consistent. The Btrfs data=ordered mode is very fast and turning it off is not required for good performance.

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