diff options
Diffstat (limited to 'Documentation/block/biodoc.txt')
| -rw-r--r-- | Documentation/block/biodoc.txt | 17 |
1 files changed, 12 insertions, 5 deletions
diff --git a/Documentation/block/biodoc.txt b/Documentation/block/biodoc.txt index 34bf8f60d8f..3adaace328a 100644 --- a/Documentation/block/biodoc.txt +++ b/Documentation/block/biodoc.txt @@ -183,7 +183,7 @@ it, the pci dma mapping routines and associated data structures have now been modified to accomplish a direct page -> bus translation, without requiring a virtual address mapping (unlike the earlier scheme of virtual address -> bus translation). So this works uniformly for high-memory pages (which -do not have a correponding kernel virtual address space mapping) and +do not have a corresponding kernel virtual address space mapping) and low-memory pages. Note: Please refer to DMA-mapping.txt for a discussion on PCI high mem DMA @@ -391,7 +391,7 @@ forced such requests to be broken up into small chunks before being passed on to the generic block layer, only to be merged by the i/o scheduler when the underlying device was capable of handling the i/o in one shot. Also, using the buffer head as an i/o structure for i/os that didn't originate -from the buffer cache unecessarily added to the weight of the descriptors +from the buffer cache unnecessarily added to the weight of the descriptors which were generated for each such chunk. The following were some of the goals and expectations considered in the @@ -403,14 +403,14 @@ i. Should be appropriate as a descriptor for both raw and buffered i/o - for raw i/o. ii. Ability to represent high-memory buffers (which do not have a virtual address mapping in kernel address space). -iii.Ability to represent large i/os w/o unecessarily breaking them up (i.e +iii.Ability to represent large i/os w/o unnecessarily breaking them up (i.e greater than PAGE_SIZE chunks in one shot) iv. At the same time, ability to retain independent identity of i/os from different sources or i/o units requiring individual completion (e.g. for latency reasons) v. Ability to represent an i/o involving multiple physical memory segments (including non-page aligned page fragments, as specified via readv/writev) - without unecessarily breaking it up, if the underlying device is capable of + without unnecessarily breaking it up, if the underlying device is capable of handling it. vi. Preferably should be based on a memory descriptor structure that can be passed around different types of subsystems or layers, maybe even @@ -946,6 +946,13 @@ elevator_merged_fn called when a request in the scheduler has been scheduler for example, to reposition the request if its sorting order has changed. +elevator_allow_merge_fn called whenever the block layer determines + that a bio can be merged into an existing + request safely. The io scheduler may still + want to stop a merge at this point if it + results in some sort of conflict internally, + this hook allows it to do that. + elevator_dispatch_fn fills the dispatch queue with ready requests. I/O schedulers are free to postpone requests by not filling the dispatch queue unless @force @@ -1013,7 +1020,7 @@ Characteristics: i. Binary tree AS and deadline i/o schedulers use red black binary trees for disk position sorting and searching, and a fifo linked list for time-based searching. This -gives good scalability and good availablility of information. Requests are +gives good scalability and good availability of information. Requests are almost always dispatched in disk sort order, so a cache is kept of the next request in sort order to prevent binary tree lookups. |