diff options
Diffstat (limited to 'docs')
| -rw-r--r-- | docs/interop/qcow2.txt | 54 | ||||
| -rw-r--r-- | docs/qcow2-cache.txt | 17 |
2 files changed, 60 insertions, 11 deletions
diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt index fb5cb47245..8c3098d8d9 100644 --- a/docs/interop/qcow2.txt +++ b/docs/interop/qcow2.txt @@ -97,7 +97,19 @@ in the description of a field. be written to (unless for regaining consistency). - Bits 2-63: Reserved (set to 0) + Bit 2: External data file bit. If this bit is set, an + external data file is used. Guest clusters are + then stored in the external data file. For such + images, clusters in the external data file are + not refcounted. The offset field in the + Standard Cluster Descriptor must match the + guest offset and neither compressed clusters + nor internal snapshots are supported. + + An External Data File Name header extension may + be present if this bit is set. + + Bits 3-63: Reserved (set to 0) 80 - 87: compatible_features Bitmask of compatible features. An implementation can @@ -126,7 +138,21 @@ in the description of a field. bit is unset, the bitmaps extension data must be considered inconsistent. - Bits 1-63: Reserved (set to 0) + Bit 1: If this bit is set, the external data file can + be read as a consistent standalone raw image + without looking at the qcow2 metadata. + + Setting this bit has a performance impact for + some operations on the image (e.g. writing + zeros requires writing to the data file instead + of only setting the zero flag in the L2 table + entry) and conflicts with backing files. + + This bit may only be set if the External Data + File bit (incompatible feature bit 1) is also + set. + + Bits 2-63: Reserved (set to 0) 96 - 99: refcount_order Describes the width of a reference count block entry (width @@ -144,10 +170,11 @@ be stored. Each extension has a structure like the following: Byte 0 - 3: Header extension type: 0x00000000 - End of the header extension area - 0xE2792ACA - Backing file format name + 0xE2792ACA - Backing file format name string 0x6803f857 - Feature name table 0x23852875 - Bitmaps extension 0x0537be77 - Full disk encryption header pointer + 0x44415441 - External data file name string other - Unknown header extension, can be safely ignored @@ -169,6 +196,16 @@ data of compatible features that it doesn't support. Compatible features that need space for additional data can use a header extension. +== String header extensions == + +Some header extensions (such as the backing file format name and the external +data file name) are just a single string. In this case, the header extension +length is the string length and the string is not '\0' terminated. (The header +extension padding can make it look like a string is '\0' terminated, but +neither is padding always necessary nor is there a guarantee that zero bytes +are used for padding.) + + == Feature name table == The feature name table is an optional header extension that contains the name @@ -437,6 +474,11 @@ L2 table entry: This information is only accurate in L2 tables that are reachable from the active L1 table. + With external data files, all guest clusters have an + implicit refcount of 1 (because of the fixed host = guest + mapping for guest cluster offsets), so this bit should be 1 + for all allocated clusters. + Standard Cluster Descriptor: Bit 0: If set to 1, the cluster reads as all zeros. The host @@ -450,8 +492,10 @@ Standard Cluster Descriptor: 1 - 8: Reserved (set to 0) 9 - 55: Bits 9-55 of host cluster offset. Must be aligned to a - cluster boundary. If the offset is 0, the cluster is - unallocated. + cluster boundary. If the offset is 0 and bit 63 is clear, + the cluster is unallocated. The offset may only be 0 with + bit 63 set (indicating a host cluster offset of 0) when an + external data file is used. 56 - 61: Reserved (set to 0) diff --git a/docs/qcow2-cache.txt b/docs/qcow2-cache.txt index c1e7751fea..d57f409861 100644 --- a/docs/qcow2-cache.txt +++ b/docs/qcow2-cache.txt @@ -158,10 +158,10 @@ refcount cache is as small as possible unless overridden by the user. Using smaller cache entries --------------------------- -The qcow2 L2 cache stores complete tables by default. This means that -if QEMU needs an entry from an L2 table then the whole table is read -from disk and is kept in the cache. If the cache is full then a -complete table needs to be evicted first. +The qcow2 L2 cache can store complete tables. This means that if QEMU +needs an entry from an L2 table then the whole table is read from disk +and is kept in the cache. If the cache is full then a complete table +needs to be evicted first. This can be inefficient with large cluster sizes since it results in more disk I/O and wastes more cache memory. @@ -172,6 +172,9 @@ it smaller than the cluster size. This can be configured using the -drive file=hd.qcow2,l2-cache-size=2097152,l2-cache-entry-size=4096 +Since QEMU 4.0 the value of l2-cache-entry-size defaults to 4KB (or +the cluster size if it's smaller). + Some things to take into account: - The L2 cache entry size has the same restrictions as the cluster @@ -185,7 +188,8 @@ Some things to take into account: - Try different entry sizes to see which one gives faster performance in your case. The block size of the host filesystem is generally a - good default (usually 4096 bytes in the case of ext4). + good default (usually 4096 bytes in the case of ext4, hence the + default). - Only the L2 cache can be configured this way. The refcount cache always uses the cluster size as the entry size. @@ -194,7 +198,8 @@ Some things to take into account: (as explained in the "Choosing the right cache sizes" and "How to configure the cache sizes" sections in this document) then none of this is necessary and you can omit the "l2-cache-entry-size" - parameter altogether. + parameter altogether. In this case QEMU makes the entry size + equal to the cluster size by default. Reducing the memory usage |