distri_024 - zhangjaycee/real_tech GitHub Wiki
4.19 内核的 erofs 是华为为安卓机开发的支持透明压缩的只读文件系统，主要针对安卓等嵌入式只读存储设备。现在主要的特点是透明压缩相对以前的“压缩前定长块”设计成了“压缩后定长块”，据说这样可以减少无用数据被解压的“读放大”，压缩算法采用的是修改过的LZ4。有个测试结果，相对ext4优势主要体现在高压缩比情况下随机读，但是存储设备只是HDD和UFS设备，并非高性能SSD设备。
- “压缩前定长块”:[1]
++-----------++-----------++ ++-----------++-----------++
...|| || || ... || || || ... original data
++-----------++-----------++ ++-----------++-----------++
\ / \ /
\ / \ /
\ / \ /
++---|-------++--|--------++ ++-----|----++--------|--++
||xxx| || |xxxxxxxx|| ... ||xxxxx| || |xx|| compressed data
++---|-------++--|--------++ ++-----|----++--------|--++
- “压缩后定长块”:[1]
|---- varient-length extent ----|------ VLE ------|--- VLE ---|
/> clusterofs /> clusterofs /> clusterofs /> clusterofs
++---|-------++-----------++---------|-++-----------++-|---------++-|
...|| | || || | || || | || | ... original data
++---|-------++-----------++---------|-++-----------++-|---------++-|
++->cluster<-++->cluster<-++->cluster<-++->cluster<-++->cluster<-++
size size size size size
\ / / /
\ / / /
\ / / /
++-----------++-----------++-----------++
... || || || || ... compressed clusters
++-----------++-----------++-----------++
++->cluster<-++->cluster<-++->cluster<-++
size size size
- 测试数据(HDD和手机UFS)[1]
Some numbers using fixed output compression (VLE, cluster size = block size = 4k) on
the server and Android phone (kirin970 platform):
Server (magnetic disk):
compression EROFS seq read EXT4 seq read EROFS random read EXT4 random read
ratio bw[MB/s] bw[MB/s] bw[MB/s] (20%) bw[MB/s] (20%)
4 480.3 502.5 69.8 11.1
10 472.3 503.3 56.4 10.0
15 457.6 495.3 47.0 10.9
26 401.5 511.2 34.7 11.1
35 389.1 512.5 28.0 11.0
48 375.4 496.5 23.2 10.6
53 370.2 512.0 21.8 11.0
66 349.2 512.0 19.0 11.4
76 310.5 497.3 17.3 11.6
85 301.2 512.0 16.0 11.0
94 292.7 496.5 14.6 11.1
100 538.9 512.0 11.4 10.8
Kirin970 (A73 Big-core 2361Mhz, A53 little-core 0Mhz, DDR 1866Mhz):
compression EROFS seq read EXT4 seq read EROFS random read EXT4 random read
ratio bw[MB/s] bw[MB/s] bw[MB/s] (20%) bw[MB/s] (20%)
4 546.7 544.3 157.7 57.9
10 535.7 521.0 152.7 62.0
15 529.0 520.3 125.0 65.0
26 418.0 526.3 97.6 63.7
35 367.7 511.7 89.0 63.7
48 415.7 500.7 78.2 61.2
53 423.0 566.7 72.8 62.9
66 334.3 537.3 69.8 58.3
76 387.3 546.0 65.2 56.0
85 306.3 546.0 63.8 57.7
94 345.0 589.7 59.2 49.9
100 579.7 556.7 62.1 57.7
* currently we use workqueue for the read-ahead process, which is still has some
minor issues and the value of sequential read is effected by work queue scheduling.
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