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f2fs File System

The Flash-Friendly File System (f2fs) was designed on the basis of a log-structured file system approach, but was modified to avoid the classical problems of the traditional log-structured approach (e.g. the snowball effect of "wandering trees" and the high "cleaning overhead").

f2fs supports various parameters not only for configuring on-disk layout but also for selecting allocation and cleaning algorithms.

System Requirements

Usage​

To format a zoned block device, that has conventional zones, with mkfs.f2fs, the option -m must be specified:

# mkfs.f2fs -m /dev/sdb

	f2fs-tools: mkfs.f2fs Ver: 1.12.0 (2018-11-12)

Info: Disable heap-based policy
Info: Debug level = 0
Info: Trim is enabled
Info: [/dev/sdb] Disk Model: HGST HSH721415AL
Info: Host-managed zoned block device:
      55880 zones, 524 randomly writeable zones
      65536 blocks per zone
Info: Segments per section = 128
Info: Sections per zone = 1
Info: sector size = 4096
Info: total sectors = 3662151680 (14305280 MB)
Info: zone aligned segment0 blkaddr: 65536
Info: format version with
  "Linux version 5.0.16-300.fc30.x86_64 (mockbuild@bkernel03.phx2.fedoraproject.org) (gcc version 9.1.1 20190503 (Red Hat 9.1.1-1) (GCC)) #1 SMP Tue May 14 19:33:09 UTC 2019"
Info: [/dev/sdb] Discarding device
Info: Discarded 14305280 MB
Info: Overprovision ratio = 0.600%
Info: Overprovision segments = 86254 (GC reserved = 43690)
Info: format successful

The formatted zoned block device can now be directly mounted. No further setup is necessary:

# mount /dev/sdb /mnt

If the zoned block device, does not have conventional zones, then a regular block device can be used for f2fs' metadata. It is formatted by using the -c option of mkfs.f2fs as shown in the following example:

# mkfs.f2fs -f -m -c /dev/nvme1n1 /dev/nvme0n1

        F2FS-tools: mkfs.f2fs Ver: 1.14.0 (2021-06-23)

Info: Disable heap-based policy
Info: Debug level = 0
Info: Trim is enabled
Info: Host-managed zoned block device:
      2048 zones, 0 randomly writeable zones
      524288 blocks per zone
Info: Segments per section = 1024
Info: Sections per zone = 1
Info: sector size = 4096
Info: total sectors = 1107296256 (4325376 MB)
Info: zone aligned segment0 blkaddr: 524288
Info: format version with
  "Linux version 5.13.0-rc6+ (user1@brahmaputra) (gcc (Ubuntu 10.3.0-1ubuntu1) 10.3.0, GNU ld (GNU Binutils for Ubuntu) 2.36.1) #2 SMP Fri Jun 18 16:45:29 IST 2021"
Info: [/dev/nvme0n1] Discarding device
Info: This device doesn't support BLKSECDISCARD
Info: This device doesn't support BLKDISCARD
Info: [/dev/nvme1n1] Discarding device
Info: Discarded 4194304 MB
Info: Overprovision ratio = 3.090%
Info: Overprovision segments = 74918 (GC reserved = 40216)
Info: format successful

In the above command, /dev/nvme1n1 is the block device file of the zoned namespace that will be used for the f2fs volume, and /dev/nvme0n1is the namespace used for the f2fs metadata.

To mount the volume formatted with the above command, the regular block device must be specified:

# mount -t f2fs /dev/nvme0n1 /mnt/f2fs/

Implementation​

Zoned Block Device Support​

Zoned block device support was added to f2fs with kernel 4.10. Because f2fs uses a metadata-block on-disk format with fixed-block location, only zoned block devices that include conventional zones are supported. Zoned devices composed entirely of sequential zones cannot be used with f2fs as a standalone device and they require a multi-device setup in order to place metadata blocks on randomly writable storage. f2fs supports multi-device setup where multiple block device address spaces are linearly concatenated to form a logically larger block device. The dm-linear device mapper target can also be used to create a logical device that is composed of both conventional zones and sequential zones suitable for f2fs.

f2fs zoned block device support was achieved using the following principles.

  1. Section Alignment In f2fs, a section is a group of fixed-size segments (2 MB). The number of segments in a section is determined to match the zoned device zone size. For example: with a 256 MB zone size, a section contains 128 segments of 2MB.
  2. Forced LFS mode By default, f2fs tries to optimize block allocation (in order to avoid excessive append write) by allowing some random writes within segments. The LFS mode forces sequential writes to segments and forces the sequential use of segments within sections, which results in full compliance with the zoned block device's write constraint.
  3. Zone reset as discard operation In the past, block discard (or trim) indicated to a device that a block or range of blocks are no longer in use. This has been replaced with the execution of a "zone write pointer reset" command when all blocks of all segments of a section are free. This allows the section to be reused.

Compared to a solution that uses the dm-zoned device mapper target, the performance of f2fs on zoned devices does not suffer from "zone reclaim overhead", because writes are always sequential and do not require on-disk temporary buffering. f2fs garbage collection (segment cleanup) generates overhead only for workloads that frequently delete files or modify files' data.

Zone Capacity Support​

SSDs with Zoned Namespace support can have a per zone capacity that is smaller than the zone size. To support such devices, f2fs ensures that block allocation and accounting considers only the blocks in a zone that are within the zone's capacity. This support for zone capacity has been available since it was introduced in Linux kernel version 5.10.

f2fs volumes need some storage space that is randomly writable in order to store and update in-place metadata blocks for the volume. Since NVMe zoned namespaces do not have conventional zones, a f2fs volume cannot be self-contained within a single NVMe zoned namespace. To format an f2fs volume using a NVMe zoned namespace, a multi-device volume format must be used in order to provide an additional regular block device to store the volume metadata blocks. This additional regular block device can be either a regular namespace on the same NVMe device or a regular namespace on another NVMe device.

Limitations​

f2fs uses 32-bit block numbers with a block size of 4 KB. This results in a maximum volume size of 16 TB. Any device or combination of devices (for a multi-device volume) with a total capacity that is larger than 16 TB cannot be used with f2fs.

To overcome this limit, the dm-linear device mapper target can be used to partition a zoned block device into serviceable, smaller logical devices. This configuration must ensure that each logical device that is created is assigned a sufficient amount of conventional zones to store f2fs fixed location metadata blocks.