Fuse Copy on Write (CoW)
Table of Contents
- Introduction
- Usage
- Implementation Details
- REST Api
Introduction
This extension of the fuse dnbd3 client makes it possible to mount images in a writable way. The changes are saved in a separate file ) on the client computer (also called Copy on Write, cow for short). These changes are uploaded to the cow server in the background. As soon as the user unmounts the image, all remaining changes are uploaded. Once all have been uploaded, the changes can be merged into a copy of the original image on the cow server (this can be set in the start parameters).
A typical use case is updating or adding software to an existing image.
Usage
New Parameters
-c <path> Enables the cow functionality. The path parameter sets the path for the temporary meta and data files in which the changes are saved.-
-C <address> sets the address of the cow server. The Cow server is responsible for merging the original image with the client's changes.
-
- L <path> Similar to -c <path>, but instead of creating a new session, an existing one is loaded from the specified path.
-
-m the client requests a merge after the image has been unmounted and all changes have been uploaded.
-
cowStatFile creates a status file at the same location as the data and meta file. The file contains information about the current session, for more information see here.
--cowStatStdout similar to --cowStatFile but the information will be printed in the stdout.
Example parameters for creating a new cow session:
./dnbd3-fuse "/home/user/VMs/mount" -f -h localhost -i imagename -c "/home/user/temp" -C "192.168.178.20:5000" --cowStatStdout -m
Implementation Details
Data structure
The data structure is divided into two main parts. The actual data of the writing on the image and the corresponding metadata. It is also important to distinguish between a dnbd3 block, which is 4096 bytes in size, and a cow block, which combines 320 dnbd3 blocks. A cow block has a cow_block_metadata_t structure that contains the corresponding metadata. The metadata is used to determine if a dnbd3 block has been written to, where that block is stored in the data file, when it was last modified and when it was uploaded. But more on this later.
Blockmetadata
TThe data structure for storing "cow_block_metadata_t" contains a layer 1 (L1) and a layer 2 (L2). L1 contains pointers to the L2's.
The entire L1 array is initialised at the beginning and cannot be resized, therefore the size of the L1 array limits the total size of the image.
The L2's are created dynamically as they are needed. So at the beginning all L1 pointers are zero. The L2's are arrays containing 1024
cow_block_metadata_t structs.
typedef struct cow_block_metadata
{
atomic_int_least64_t offset;
atomic_uint_least64_t timeChanged;
atomic_uint_least64_t uploads;
atomic_char bitfield[40];
} cow_block_metadata_t;
Each cow_block_metadata_t contains a 40 byte, 320 bit bit field. The bit field indicates whether the corresponding dnbd3 block contains data or not. If, for example, the bit field begins with 01... the first 4096 contain no data and the next 4096 contain data.
So each cow_block_metadata_t stores the metadata of up to 320*4096 bytes if all bits are set to 1. The offset field is the offset where in the data file the corresponding data is stored. The property timeChanged contains the Unix time when the block was last changed. It is 0 if it has never been changed or if the last changes have already been uploaded.
The L2 arrays and cow_block_metadata_t are sorted according to the original offsets of the image. Thus the first L1 pointer, i.e. the first L2 array, addresses the first 1024 * 320 * 4096 bytes (L2Size * bitfieldsize * DNBD3Blocksize) of data and so on.
For example, to get the "cow_block_metadata_t" for offset 4033085440, one would take L1[3], since
4033085440 / ( COW_L2_STORAGE_CAPACITY ) ≈ 3.005
Then one would take the fifth cow_block_metadata_t in the L2 array because of
(4033085440 mod COW_L2_STORAGE_CAPACITY) / COW_METADATA_STORAGE_CAPACITY = 5
Where:
COW_L2_STORAGE_CAPACITY = 1024 * 320 * 4096
COW_METADATA_STORAGE_CAPACITY = 320 * 4096
Read Request
When a read request is made, it is checked for each 4096-byte block whether the block already exists locally on the computer (i.e. has already been written once). If so, it is read from the hard disk, otherwise it is requested from the dnbd3 server. To increase performance, several subsequent blocks that are also local/non-local are combined into a larger read from the hard disk or request from the server.
The diagram above is somewhat simplified for clarity. The server's read operations are asynchronous. This means that it does not wait for a response from the server, but continues with the next blocks. As soon as the server's response is complete, the data is written to the fuse buffer.
Each request to the dnbd3 server increases the variable workCounter by one, and each time a request is completed, it is decreased by one. As soon as workCounter is 0 again, fuse_request is returned. This is done to ensure that all asynchronous requests are completed before the request is returned.
For local blocks, the loop must be interrupted as soon as the end of a cow_block_metadata_t is reached, as the next data offset of the next cow_block_metadata_t is most likely not directly after it in the data file.
Write Request
If, in a write request, the beginning or end does not match a multiple of 4096, the beginning and/or end block must be padded.
This is because each 4096-byte block requires complete data, because if the bit in the bit field for that block is set, all data is read locally.
To fill the block, if it is still within the range of the original image size, the missing bytes are requested from the dnbd3 server. If it is outside the original image size (because the image has become larger), the missing bytes are filled with 0.
The write request calculates the corresponding cow_block_metadata_t from the offset. If the corresponding cow_block_metadata_t does not yet exist, it is created. The data will be written to the data file, at the offset stored in the cow_block_metadata_t.
Then the corresponding bit in the bit fields is set and the timeChanged is updated. If there is more data to write, the next cow_block_metadata_t is calculated and the above steps are repeated.
The variable workCounter is also used here to ensure that the padding of the data occurs before the Fuse request returns.
Block Upload
For uploading blocks, there is a background thread that periodically loops over all Cow blocks and checks whether timeChanged is not 0 and the time difference between now and timeChanged is greater than COW_MIN_UPLOAD_DELAY. If so, the block is uploaded. The timeChanged before the upload is buffered. After the upload, timeChanged is set to 0 if it still has the same time as the temporarily stored one (if not, there was a change during the upload and it has to be uploaded again). Once the image is unmounted, COW_MIN_UPLOAD_DELAY is ignored and all blocks that have a time other than 0 are uploaded. The upload is done via a rest request. There are two different limits for the number of parallel uploads in the config.h.
Files
When a new CoW session is started, a new meta, data and, if so set in the command line arguments, a status.txt file is created.
status
The file status.txt can be activated with the command line parameter --cowStatFile.
The file will contain the following:
uuid=<uuid>
state=backgroundUpload
inQueue=0
modifiedBlocks=0
idleBlocks=0
totalBlocksUploaded=0
activeUploads:0
ulspeed=0.00
-
The uuid is the session uuid used by the Cow server to identify the session.
-
The status is backgroundUpload when the image is still mounted and cow blocks are uploaded in the background.
It is uploading when the image has been unmounted and all blocks that have not yet been uploaded are uploaded.
It is done when the image has been unmounted and all blocks have been uploaded.
Queue are the cow blocks that are currently being uploaded or are waiting for a free slot.ModifiedBlocks are cow blocks that have changes that have not yet been uploaded to the server because the changes are too recent.totalBlocksUploaded the total amount of cow blocks uploaded since the image was mounted.activeUploads is the number of blocks currently being uploaded.ulspeed the current upload speed in kb/s.
Once all blocks have been uploaded, the status is set to done.
If you define COW_DUMP_BLOCK_UPLOADS, a list of all blocks, sorted by the number of uploads, is copied to the status.txt file after the block upload is completed.
With the command line parameter --cowStatStdout the same output of the stats file will be printed in stdout.
meta
The meta file contains the following header:
// cowfile.h
typedef struct cowfile_metadata_header
{
uint64_t magicValue; // 8byte
atomic_uint_least64_t imageSize; // 8byte
int32_t version; // 4byte
int32_t blocksize; // 4byte
uint64_t originalImageSize; // 8byte
uint64_t metaDataStart; // 8byte
int32_t bitfieldSize; // 4byte
int32_t nextL2; // 4byte
atomic_uint_least64_t metadataFileSize; // 8byte
atomic_uint_least64_t dataFileSize; // 8byte
uint64_t maxImageSize; // 8byte
uint64_t creationTime; // 8byte
char uuid[40]; // 40byte
char imageName[200]; // 200byte
} cowfile_metadata_header_t;
After this header, the above-mentioned l1 and then the l2 data structure begins at byte 8192.
data
The data files contain the magicValue and at the 40 * 8 * 4096 offset (capacity of a cowfile_metadata_header_t) the first data block starts.
magic values in the file headers
The magic values in both files are used to ensure that an appropriate file is read and that the machine has the correct endianness.
//config.h
#define COW_FILE_META_MAGIC_VALUE ((uint64_t)0xEBE44D6E72F7825E) // Magic Value to recognize a Cow meta file
#define COW_FILE_DATA_MAGIC_VALUE ((uint64_t)0xEBE44D6E72F7825F) // Magic Value to recognize a Cow data file
Threads
This extension uses two new threads:
tidCowUploader
tidStatUpdater
tidCowUploader is the thread that uploads blocks to the cow server.
tidStatUpdater updates the stats in stdout or the stats files (depending on parameters).
Locks
This extension uses a new lock cow.l2CreateLock. It is used when a new L2 array is allocated.
Config Variables
The following configuration variables have been added to config.h.
//config.h
// +++++ COW +++++
#define COW_BITFIELD_SIZE 40 // NEVER CHANGE THIS OR THE WORLD WILL ALSO END!
#define COW_FILE_META_MAGIC_VALUE ((uint64_t)0xEBE44D6E72F7825E) // Magic Value to recognize a Cow meta file
#define COW_FILE_DATA_MAGIC_VALUE ((uint64_t)0xEBE44D6E72F7825F) // Magic Value to recognize a Cow data file
#define COW_MIN_UPLOAD_DELAY 60 // in seconds
#define COW_STATS_UPDATE_TIME 5 // time in seconds the cow status files gets updated (while uploading blocks)
#define COW_MAX_PARALLEL_UPLOADS 10 // maximum number of parallel uploads
#define COW_MAX_PARALLEL_BACKGROUND_UPLOADS 2 // maximum number of parallel uploads while the image is still mounted
#define COW_URL_STRING_SIZE 500 // Max string size for an url
#define COW_SHOW_UL_SPEED 1 // enable display of ul speed in cow status file
#define COW_MAX_IMAGE_SIZE 1000LL * 1000LL * 1000LL * 1000LL; // Maximum size an image can have(tb*gb*mb*kb)
// +++++ COW API Endpoints +++++
#define COW_API_CREATE "%s/api/File/Create"
#define COW_API_UPDATE "%s/api/File/Update?guid=%s&BlockNumber=%lu"
#define COW_API_START_MERGE "%s/api/File/StartMerge"
COW_MIN_UPLOAD_DELAY sets the minimum time in seconds that must have elapsed since the last change to a cow block before it is uploaded. This value can be fine-tuned. A larger value usually reduces the double uploading of blocks. A smaller value reduces the time for the final upload after the image has been unmounted. If you set COW_DUMP_BLOCK_UPLOADS and set the command line parameter --cowStatFile, then a list of all blocks, sorted by the number of uploads, will be written to the status.txt file after the block upload is complete. This can help in fine-tuning COW_MIN_UPLOAD_DELAY.COW_STATS_UPDATE_TIME defines the update frequency of the stdout output/statistics file in seconds. Setting it too low could affect performance as a loop runs over all blocks.COW_MAX_PARALLEL_BACKGROUND_UPLOADS defines the maximum number of parallel block uploads. This number is used when the image is still mounted and the user is still using it.COW_MAX_PARALLEL_UPLOADS defines the maximum number of parallel block uploads. This number is used once the image has been unmounted to upload the remaining modified blocks.
REST Api
The following Rest API is used to transmit the data and commands to the cow server:
/api/File/Create
POST
Responses
| Code |
Description |
| 200 |
Success |
This request is used as soon as a new cow session is created. The returned guid is used in all subsequent requests to identify the session.
/api/File/Update
POST
Parameters
| Name |
Located in |
Description |
Required |
Schema |
| guid |
query |
|
Yes |
string (uuid) |
| blockNumber |
query |
|
Yes |
integer |
Responses
| Code |
Description |
| 200 |
Success |
Used to upload a data block. The block number is the absolute block number. The body contains an "application/octet-stream", where the first bytes are the bit field, directly followed by the actual block data.
/api/File/StartMerge
POST
Parameters
| Name |
Located in |
Description |
Required |
Schema |
| guid |
Form |
|
Yes |
string (uuid) |
| originalFileSize |
Form |
|
Yes |
integer |
| newFileSize |
Form |
|
Yes |
integer |
| ##### Responses |
|
|
|
|
| Code |
Description |
| 200 |
Success |
| Used to start the merge on the server. |
|
/api/File/GetTopModifiedBlocks
GET
Parameters
| Name |
Located in |
Description |
Required |
Schema |
| guid |
query |
|
Yes |
string (uuid) |
| amount |
query |
|
Yes |
integer |
Responses
| Code |
Description |
| 200 |
Success |
This request returns a list containing the block IDs and the number of uploads of this block, sorted by the number of uploads. This is useful to adjust the COW_MIN_UPLOAD_DELAY.
/api/File/Status
GET
Parameters
| Name |
Located in |
Description |
Required |
Schema |
| guid |
query |
|
Yes |
string (uuid) |
Responses
| Code |
Description |
| 200 |
Success |
Returns the SessionStatus model that provides information about the session.
Models
BlockStatistics
| Name |
Type |
Description |
Required |
| blockNumber |
integer |
|
Yes |
| modifications |
integer |
|
Yes |
SessionState
| Name |
Type |
Description |
Required |
| SessionState |
string |
|
|
SessionStatus
| Name |
Type |
Description |
Required |
| state |
string |
Enum: "Copying", "Active", "Merging", "Done", "Failed" |
Yes |
| imageName |
string |
|
Yes |
| originalImageVersion |
integer |
|
Yes |
| newImageVersion |
integer |
|
Yes |
| mergedBlocks |
integer |
|
Yes |
| totalBlocks |
integer |
|
Yes |
