Revision as of 01:26, 28 April 2011

Introduction

This article describes how various different ext4 write requests are handled in Linux 2.6.34 -- 2.6.39. Special attention will be paid to how ext4_map_blocks() is called, and with what EXT4_GET_BLOCKS_* flags. In addition how quota is reserved, claimed, and released will be discussed.

Flags passed to ext4_map_blocks()

The primary function of the ext4_map_blocks() function is to translate an inode and logical block number to a physical block number. If a mapping between a particular logical block number and physical block number does not exist, ext4_map_blocks() may create such a mapping, allocating blocks as necessary. However, there are a number of other things which ext4_map_blocks() can do, based on a flags bitmap which is passed to it. In some cases, a particular flag to ext4_map_blocks() can radically change its behavior. So it's important to document the current ext4_map_blocks flags and what they do.

At one point, ext4_map_blocks() had previously been called ext4_get_blocks(), which is the reason for the naming convention of these flags:

EXT4_GET_BLOCKS_CREATE 

to be written

EXT4_GET_BLOCKS_UNINIT_EXT 

to be written

EXT4_GET_BLOCKS_CREATE_UNINIT_EXT 

to be written

EXT4_GET_BLOCKS_DELALLOC_RESERVE 

to be written

EXT4_GET_BLOCKS_PRE_IO 

to be written

EXT4_GET_BLOCKS_CONVERT 

to be written

EXT4_GET_BLOCKS_IO_CREATE_EXT 

to be written

EXT4_GET_BLOCKS_IO_CONVERT_EXT 

to be written

Life of a nodelalloc buffered write

The write request

Description of how a write request happens from userspace (i.e., the codepath from generic_file_buffered_write() calling ext4_write_begin() and ext4_{writeback,ordered,journalled}_write_end() and/or the codepath from page_mkwrite() calling ext4_page_mkwrite()).

I/O submission

What happens when generic_writepages() calls ext4_write_cache_pages() which then calls ext4_writepage()

Life of a delalloc buffered write

The write request

Description of how a write request happens from userspace (i.e., the codepath from generic_file_buffered_write() calling ext4_da_write_begin() and ext4_da_write_end() and/or the codepath from page_mkwrite() calling ext4_page_mkwrite()).

I/O submission

What happens in ext4_writepages()

Modifications with dioread_nolock

What changes if dioread_nolock is enabled.

Life of a direct I/O write

Life of an async direct I/O write