ext4 文件系统的一些记录

https://www.kernel.org/doc/Documentation/filesystems/ext4.txt

ext4 权威说明

http://computer-forensics.sans.org/blog/2010/12/20/digital-forensics-understanding-ext4-part-1-extents#

对ext4的extent有解释，有实际查看；

http://blog.csdn.net/evilcode/article/details/7006925

ext4的nodealloc相关分析

http://us.generation-nt.com/answer/ext4-data-writeback-performs-worse-than-data-ordered-now-help-205753041.html

在一些特定的情况下，writeback性能还会差，有人怀疑是众多小块写，也可能是磁盘个数多，nr_request太大导致其他资源不足。

关于writeback vs order(http://stackoverflow.com/questions/12845319/ordered-mode-vs-writeback-mode)

If we don't go much in detail , then the answer to the query is yes (at least in filesystem like ext3).In both the modes only filesystem metadata is written into the journal. The difference between writeback mode and ordered mode is that in Ordered mode filesystem groups metadata and relative data blocks so that data blocks are written to disk before the metadata is logged while in ordered only metadata is logged (& data blocks are not written into disk at all).

ii. From the implementation perspective(in terms of ext3), in ordered mode an additional function journal_dirty_data( ) is invoked on every buffer of data in the page to insert the buffer in a proper list of the active transactions. The JBD layer ensures that all buffers in this list are written to disk before the metadata buffers of the transaction.After that generic_commit_write( ) function is invoked, which inserts the data buffers in the list of the dirty buffers of the owner inode. In writeback mode , no such function like journal_dirty_data( ) is invoked on data buffers and only generic_commit_write( ) is called.

关于barrier(http://baike.baidu.com/view/2220807.htm)

磁 盘上配有内部缓存，以便重新调整批量数据的写操作顺序，优化写入性能，因此文件系统必须在日志数据写入磁盘之后才能写 commit 记录，若 commit 记录写入在先，而日志有可能损坏，那么就会影响数据完整性。Ext4 默认启用 barrier，只有当 barrier 之前的数据全部写入磁盘，才能写 barrier 之后的数据。（可通过 "mount -o barrier=0" 命令禁用该特性。）

-o discard 利于SSD类TRIM功能，但是看官方文档貌似认为还需要更多测试，到2.6.37似乎是比较稳定了。https://patrick-nagel.net/blog/archives/337

不用discard因为在频繁删除小文件的时候，sync会比较长时间；在系统清闲的时候调用fstrim可能更好；不过看起来fstrim不像是多么稳妥的方式，企业环境下还是不要用为好，借助SSD自己的驱动来做就好了。