com.orientechnologies.orient.core.storage.cache

Interface OReadCache

All Known Implementing Classes:

O2QCache, ODirectMemoryOnlyDiskCache

public interface OReadCache

This class is heart of OrientDB storage model it presents disk backed data cache which works with direct memory.

Model of this cache is based on page model. All direct memory area is mapped to disk files and each file is split on pages. Page is smallest unit of work. The amount of RAM which can be used for data manipulation is limited so only a subset of data will be really loaded into RAM on demand, if there is not enough RAM to store all data, part of them will by flushed to the disk. If disk cache is closed all changes will be flushed to the disk.

Typical steps if you work with disk cache are following:

1. Open file using #openFile(String, OWriteCache) method
2. Remember id of opened file
3. Load page which you want to use to write data using method #load(long, long, boolean, OWriteCache, int)
4. Get pointer to the memory page OCacheEntry.getCachePointer()
5. Lock allocated page for writes OCachePointer.acquireExclusiveLock()
6. Get pointer to the direct memory which is allocated to hold page data OCachePointer#getDataPointer()
7. Change page content as you wish.
8. Release page write lock OCachePointer.releaseExclusiveLock()
9. Mark page as dirty so it will be flushed eventually to the disk OCacheEntry.markDirty()
10. Put page back to the cache release(OCacheEntry, OWriteCache)

If you wish to read data, not change them, you use the same steps but:

1. Acquire read lock instead of write lock using OCachePointer.acquireSharedLock()
method.
2. Do not mark page as dirty

If you want to add new data but not to change existing one and you do not have enough space to add new data use method #allocateNewPage(long, OWriteCache) instead of #load(long, long, boolean, OWriteCache, int).

#load(long, long, boolean, OWriteCache, int) method has checkPinnedPages parameter. Pinned pages are pages which are kept always loaded in RAM ,this class of pages is needed for some data structures usually this attribute should be set to false and it is set to true when storage goes through data restore procedure after system crash.

Since:

14.03.13

Author:

Andrey Lomakin

Method Summary

Modifier and Type	Method and Description
long	addFile(String fileName, long fileId, OWriteCache writeCache)
long	addFile(String fileName, OWriteCache writeCache)
OCacheEntry	allocateNewPage(long fileId, OWriteCache writeCache, boolean verifyChecksums)
void	clear()
void	closeFile(long fileId, boolean flush, OWriteCache writeCache)
void	closeStorage(OWriteCache writeCache) Closes all files inside of write cache and flushes all associated data.
void	deleteFile(long fileId, OWriteCache writeCache)
void	deleteStorage(OWriteCache writeCache)
long	getUsedMemory()
OCacheEntry	load(long fileId, long pageIndex, boolean checkPinnedPages, OWriteCache writeCache, int pageCount, boolean verifyChecksums)
void	loadCacheState(OWriteCache writeCache) Load state of cache from file system if possible.
void	pinPage(OCacheEntry cacheEntry)
void	release(OCacheEntry cacheEntry, OWriteCache writeCache)
void	storeCacheState(OWriteCache writeCache) Stores state of cache inside file if possible.
void	truncateFile(long fileId, OWriteCache writeCache)

Method Detail

addFile

long addFile(String fileName,
             OWriteCache writeCache)
      throws IOException

Throws:

IOException

addFile

long addFile(String fileName,
             long fileId,
             OWriteCache writeCache)
      throws IOException

Throws:

IOException

load

OCacheEntry load(long fileId,
                 long pageIndex,
                 boolean checkPinnedPages,
                 OWriteCache writeCache,
                 int pageCount,
                 boolean verifyChecksums)
          throws IOException

Throws:

IOException

pinPage

void pinPage(OCacheEntry cacheEntry)
      throws IOException

Throws:

IOException

allocateNewPage

OCacheEntry allocateNewPage(long fileId,
                            OWriteCache writeCache,
                            boolean verifyChecksums)
                     throws IOException

Throws:

IOException

release

void release(OCacheEntry cacheEntry,
             OWriteCache writeCache)

getUsedMemory

long getUsedMemory()

clear

void clear()

truncateFile

void truncateFile(long fileId,
                  OWriteCache writeCache)
           throws IOException

Throws:

IOException

closeFile

void closeFile(long fileId,
               boolean flush,
               OWriteCache writeCache)
        throws IOException

Throws:

IOException

deleteFile

void deleteFile(long fileId,
                OWriteCache writeCache)
         throws IOException

Throws:

IOException

deleteStorage

void deleteStorage(OWriteCache writeCache)
            throws IOException

Throws:

IOException

closeStorage

void closeStorage(OWriteCache writeCache)
           throws IOException

Closes all files inside of write cache and flushes all associated data.

Parameters:

writeCache - Write cache to close.

Throws:

IOException

loadCacheState

void loadCacheState(OWriteCache writeCache)

Load state of cache from file system if possible.

Parameters:

writeCache - Write cache is used to load pages back into cache if possible.

storeCacheState

void storeCacheState(OWriteCache writeCache)

Stores state of cache inside file if possible.

Parameters:

writeCache - Write cache which manages files cache state of which is going to be stored.