ploop

NAME

ploop − ploop device management utility

SYNOPSYS

[-u uuid] DiskDescriptor.xml

ploop snapshot

[-H] [-u uuid] [-s] [-o field[,field...]] DiskDescriptor.xml

ploop balloon discard

ploop restore-descriptor

DESCRIPTION

ploop is a kernel block device, similar to the traditional loop device (which is controlled by losetup(8)) but with more features added, such as dynamic disk space allocation, stackable images, online resize, snapshotting, and live migration helper (write tracker). This manual page describes the ploop user space tool which is used to perform various operations related to ploop devices and images.

Note that this ploop tool is not aware of container entities. Commands that have DiskDescriptor.xml as an argument work with an XML file that contains meta-information about a particular ploop device configuration: device characteristics (block size etc.), storage information (names and formats of images used for the device), snapshot information, etc. If a particular command can be used both with and without the DiskDescriptor.xml argument, it is strongly advised to use the form with DiskDescriptor.xml.

OPTIONS

Run ploop without any options to show a short synopsys, including a list of commands.

Run ploop command to show synopsys and a short description for a particular command.

Basic commands

init

Create and initalize a ploop image file and a corresponding DiskDescriptor.xml file.

-t none|ext3|ext4

File system type to create, default is ext4. Unless none is specified, a partition, a filesystem, and a balloon file will be created inside the image. Using ext3 is not recommended.

-b blocksize

Device block size, in 512 byte sectors. Default block size is 2048 sectors, or 1 megabyte.

-B fsblocksize

Filesystem block size, in bytes. Default is 4096 bytes.

-n, --nolazy

Disable lazy mkfs initialization (which is enabled by default). Currently that means if this flag is set, mkfs.ext4(8) is called with -Elazy_itable_init=0,lazy_journal_init=0 options.

mount

Assemble a ploop device from one or more delta images, start it, and optionally mount the file system residing on the device.

Two forms of this command are provided. The first one accepts a list of delta images to be used for assembling the ploop device, while the second one is using information from a DiskDescriptor.xml file. Please note that not all mount options are applicable to both forms.

-b blocksize

Device block size, in 512 byte sectors. Ignored if DiskDescriptor.xml is specified. Otherwise, required for raw images.

-m mount_point

If this option is specified, ploop goes on to mount the file system to directory denoted by mount_point.

-o mount_options

Any additional mount options, comma-separated. Used if -m is set.

-u uuid | base

GUID of the image from the DiskDescriptor.xml to be mounted. By default, top GUID is used. The special ’base’ value can be used to mount the base (lower-level) image.

base_delta [... top_delta]

List of image files to mount, with the first one being the base delta and the last one being the top delta (i.e. the one that will be writable unless -r is specified).

DiskDescriptor.xml

Path to the DiskDescriptor.xml file with information about images.

umount

Unmount a ploop device. Since a mounted ploop device consists of an image (or multiple images), a device, and (optionally) a file system mounted to a directory, one can refer to any of the above entities to specify what to unmount. The recommended way is to use DiskDescriptor.xml.

-m mount_point

Mount point of a ploop device to unmount.

DiskDescriptor.xml

Path to the DiskDescriptor.xml file with information about images.

replace

Replaces a ploop image by a different (but identical) one, on a running ploop device. Only a read-only image (e.g. a non-top one in a stacked configuration) can be replaced. An image to be replaced is specified by either one of level, UUID, or the current image file.

If a new image is not identical to the old one (i.e. its content differs) or not suitable for ploop in any other way (e.g. it is sparse, or resides on a file system not supported by ploop), the result is undefined.

-o cur_image_file

A current image file (the one to replace).

-k, --keep-name

A flag to keep the same image file name. If this flag is set, after the image is replaced, a new image_file is renamed to the old one (removing the old, now unused, image), so no modification to DiskDescriptor.xml is required.

DiskDescriptor.xml

Path to the DiskDescriptor.xml file with information about images.

resize

Resize a ploop image. Both online (i.e. when ploop is mounted and used) and offline resize is supported, and the tool can either grow or shrink both the ploop image and the underlying file system.

DiskDescriptor.xml

Path to the DiskDescriptor.xml file with information about images.

convert

Convert either ploop image format or version (but not both at the same time). Conversion can only be performed offline (i.e. image should not be in use).

check

Check the internal consistency of (and possibly repair) a ploop image (or images). Note that image(s) to be tested should not be in use.

Check all the images in DiskDescriptor.xml up to the one denoted by the uuid (or default top delta, if UUID is not specified). Default built-in check options are used, and the ones specified on the command line, if any, are ignored.

-F, --hard-force

Same as -f, plus try to fix even fatal errors (can be dangerous).

-s, --silent

Be more silent, only report errors.

-d, --drop-inuse

Drop image "in use" flag.

-b, --blocksize size

Image cluster block size, in sectors (for raw images).

-S, --repair-sparse

Repair sparse image(s).

Miscellaneous commands

info

When run without any options, show information about disk space and inodes usage and limits on the inner ploop filesystem, somewhat similar to vzquota(8) stat or show commands.

Either one or both options can be used together. Option -s is used to show information about ploop device size, block size, and format version. Option -d is used to show a corresponding ploop block device, it available. file.

list

Shows a list of running ploop devices (first column) and their corresponding base images. With option -a it also shows a mount point (third column).

Working with snapshots
Ploop snapshots is a mechanism for creating and managing instant states of a running file system. Creating a snapshot leads to creating a new empty ploop image which is layered on top of an old one, then all writes are ending up in the top image, and reads are falling through to a lower level. There can be up to 126) stacked ploop images (or snapshots). Online snapshot merging is also supported.

Snapshots are identified by a unique UUID. A snapshot can be mounted using ploop mount -u uuid command, see above.

snapshot

Create a ploop snapshot.

snapshot-merge

Merge a snapshot with its parent.

snapshot-switch

Switch to the specified snapshot. This operation can only be performed while ploop is not running (i.e. is unmounted). The current top delta image will be removed.

snapshot-delete

Delete the specifed snapshot. This operation can only be performed if the specified snapshot is not active. In case snapshot doesn’t have any children, it will simply be removed. In case snapshot has a single child, it will be merged to that child. Deleting a snapshot that has multiple children is currently not supported (but can be performed manually in an iterative fashion).

snapshot-list

List available snapshots.

-u, --uuid, --id uuid

Filter the output to a specified uuid.

-o, --output field[,field...]

Display only the specified fields. Possible fields are:
• uuid - snapshot’s UUID;
• parent_uuid - snapshot’s parent UUID;
• current - if this snapshot is the current one;
• fname - snapshot image file name.

Image copying
ploop copy is a mechanism of effective copying of a top ploop image with the help of build-in ploop kernel driver feature called write tracker. Write tracker is a feature that lets ploop copy to iteratively obtain a list of modified image blocks from the kernel. Two ploop copy processes are required for iterative top delta transfer. These are used by vzmigrate(8).

copy (sending)

This command enables the in-kernel write tracker for the specified ploop device, then sends all the data blocks from the top delta image to a pipe specified by the output_fd argument (stdout, i.e. 1 by default), supposedly read by destination ploop copy, or a file. After that, it iteratively gets the list of the modified data blocks from the kernel and sends those blocks again. After a number of iterations (or when the list is empty), it executes the stop_command (this could be vzctl stop or vzctl chkpnt) and does the last iteration of sending the modified data blocks. Finally, it checks that the data were not modified, error is returned otherwise.

If feedback_fd is specified, it is used to read back from the ploop copy receiving side. The feedback channel is currently used to wait for fdatasync(2) completion.

copy (receiving)

Reads the data blocks (provided by the source ploop copy) from the file descriptor input_fd (stdin, i.e. 0 by default) and writes them to the file.

If feedback_fd is specified, it is used to send status back to the ploop copy sending side.

Ballooning
Since there is no online shrink support in ext4 file system, ploop internally uses a technique called "ballooning" as a work around to shrink its images.

Ballooning operation consists of inflating a special balloon file (invisible for ordinary users), loading fiemap info of the inflated balloon to the kernel, relocating blocks of the image file from the tail to the space specified by fiemap info, and truncating the tail of the image file. Result is the image file of a smaller size.

However, it is quite possible that inflated balloon file will only span blocks that were never touched before. Those will look like "not allocated" space from the kernel ploop point of view. In this case nothing will be relocated and nothing truncated.

So, if balloon operation succeeded, it’s only guaranteed that a user of ploop device won’t be able to consume more space than the initial block device size minus the size of the inflated balloon. On the other hand, if a user of block device used a lot of space on it, then freed the significant part of used space, balloon operation will result in significant truncate of image file.

All the ploop ballooning logic is hidden from the end user, so while a number of low-level commands exist for working with ploop ballooning, those are not needed and therefore are not documented here, except for a single command.

balloon discard

In a situation when a lot of disk space were freed on an in-ploop filesystem, use ploop balloon discard to optimize the ploop image size.

Iteratively try to relocate and discard unused blocks from a ploop image, reducing its size.

Note that ploop device and its inner file system should be mounted. If not, one can use --automount option to automatically mount ploop for the duration of the operation.

Option --to-free can be used to specify a maximum disk space to be freed. In other words, stop the process once freed space exceeded requested size. Default is 0, meaning to try to free as much space as possible.

Option --min-block can be used to specify a minimum size of an extent to free. The smallest possible extent is 1 cluster (currently 1 MB), one can specify higher value to speed up the whole discarding operation.

Note that the same functionality is available by means of vzctl compact command.

restore descriptor

Create DiskDescriptor.xml file suitable for delta_file and put it into disk_dir.

Read image header in case of ploop1 format or check raw image size and generate proper DiskDescriptor.xml file. You can specify blocksize for raw images. If it’s not specified it will be choosen automatically - largest possible value between 32K and 1M. Raw image size must be aligned to blocksize.

This command works only for base images. Snapshots are not supported.

Image formats
The following image formats are currently supported.

ploop1, expanded

Expanded format. The image will grow according to the needs of the underlying file system. This format is the default. Names ’ploop1’ and ’expanded’ are aliases.

preallocated

This is the same as ’ploop1’ or ’expanded’, the only difference is all the file blocks are allocated during creation.

EXIT STATUS

ploop exits with status 0 in case of successful execution. Any status greater than 0 signifies an error.
1, SYSEXIT_CREAT

Error creating a file.

2, SYSEXIT_DEVICE

Error getting or opening a ploop device.

3, SYSEXIT_DEVIOC

Error doing ioctl(2) on ploop device.

4, SYSEXIT_OPEN

Error opening a file.

5, SYSEXIT_MALLOC

Not enough memory (error from malloc(3), realloc(3), calloc(3), or posix_memalign(3)).

6, SYSEXIT_READ

Error during read.

7, SYSEXIT_WRITE

Error during write.

9, SYSEXIT_SYSFS

Error reading from a sysfs file (usually under /sys/block/ploop...).

11, SYSEXIT_PLOOPFMT

Corrupted ploop image detected.

12, SYSEXIT_SYS

Other system error.

13, SYSEXIT_PROTOCOL

Broken protocol (unexpected value received).

14, SYSEXIT_LOOP

pcopy command can’t finalize copying (frozen filesystem is changing).

15, SYSEXIT_FSTAT

Error from stat(2), fstat(2), or statfs(2).

16, SYSEXIT_FSYNC

Error from fsync(2) or syncfs(2).

17, SYSEXIT_EBUSY

Can’t continue, another operation is in progress.

18, SYSEXIT_FLOCK

Error from flock(2).

19, SYSEXIT_FTRUNCATE

Error from ftruncate(2) or truncate(2).

20, SYSEXIT_FALLOCATE

Error from fallocate(2).

21, SYSEXIT_MOUNT

Can’t mount ploop image or file system.

22, SYSEXIT_UMOUNT

Can’t unmount ploop image or file system.

23, SYSEXIT_LOCK

Locking failed (another operation in progress?).

24, SYSEXIT_MKFS

Can’t create file system.

26, SYSEXIT_RESIZE_FS

Utility resizefs failed.

27, SYSEXIT_MKDIR

Error from mkdir(2).

28, SYSEXIT_RENAME

Error from rename(2).

29, SYSEXIT_ABORT

Operation aborted.

30, SYSEXIT_RELOC

Block relocation failed.

33, SYSEXIT_CHANGE_GPT

Error resizing GPT partition table.

35, SYSEXIT_UNLINK

Error from unlink(2).

36, SYSEXIT_MKNOD

Error from mknod(2).

37, SYSEXIT_PLOOPINUSE

Image is already in use.

38, SYSEXIT_PARAM

Invalid parameter.

39, SYSEXIT_DISKDESCR

Problem with DiskDescriptor.xml file.

40, SYSEXIT_DEV_NOT_MOUNTED

Ploop image is not mounted.

41, SYSEXIT_FSCK

Error from fsck(8).

SEE ALSO

vzctl(8), vzmigrate(8), http://openvz.org/Ploop.