移植facebook flashcache模块支持 linux kernel 4.13以上
环境
准备在ubuntu 18.04 lts下搭建kvm+libvirt+glusterfs的虚拟化环境。
4台服务器是2块SSD，4块SATA（5400RPM）。
尝试了gluster tier，效果不好。
对于虚拟化环境下大文件的存储，tier的promote和demote的策略很难设置，
在大文件发生大量数据交换时会出现tier层"no space left"的错误，文件无法写入。
在没有更好的解决方案之前，facebook的flashcache（https://github.com/facebookarchive/flashcache )对于提速SATA盘的作用还是非常显著的。
但ubuntu18.04的apt源还没有。
https://launchpad.net/flashcache/+packages
错误
于是下载flashcache源码编译dkms模块的时候，报错：
DKMS make.log for flashcache-3.1.3+git20150701 for kernel 4.15.0-36-generic (x86_64)
Fri Oct 12 15:06:00 UTC 2018
make -C /lib/modules/4.15.0-36-generic/build M=/var/lib/dkms/flashcache/3.1.3+git20150701/build modules V=0
make[1]: Entering directory '/usr/src/linux-headers-4.15.0-36-generic'
Makefile:975: "Cannot use CONFIG_STACK_VALIDATION=y, please install libelf-dev, libelf-devel or elfutils-libelf-devel"
CC [M] /var/lib/dkms/flashcache/3.1.3+git20150701/build/flashcache_conf.o
CC [M] /var/lib/dkms/flashcache/3.1.3+git20150701/build/flashcache_main.o
CC [M] /var/lib/dkms/flashcache/3.1.3+git20150701/build/flashcache_subr.o
/var/lib/dkms/flashcache/3.1.3+git20150701/build/flashcache_subr.c: In function ‘flashcache_bio_endio’:
/var/lib/dkms/flashcache/3.1.3+git20150701/build/flashcache_subr.c:742:7: error: ‘struct bio’ has no member named ‘bi_error’; did you mean ‘bi_iter’?
bio->bi_error = error;
^~~~~~~~
bi_iter
scripts/Makefile.build:332: recipe for target '/var/lib/dkms/flashcache/3.1.3+git20150701/build/flashcache_subr.o' failed
make[2]: *** [/var/lib/dkms/flashcache/3.1.3+git20150701/build/flashcache_subr.o] Error 1
Makefile:1551: recipe for target '_module_/var/lib/dkms/flashcache/3.1.3+git20150701/build' failed
make[1]: *** [_module_/var/lib/dkms/flashcache/3.1.3+git20150701/build] Error 2
make[1]: Leaving directory '/usr/src/linux-headers-4.15.0-36-generic'
Makefile:35: recipe for target 'modules' failed
make: *** [modules] Error 2OS内核的版本是4.15.0-36。
原因
根据错误提示：bio->bierror ‘struct bio’ has no member named ‘bierror’;
找了4.15的内核源码看看bio的struct的定义,
bio是在include/linux/blktypes.h中定义。
https://elixir.bootlin.com/linux/v4.15/source/include/linux/blktypes.h
struct bio {
struct bio *bi_next; /* request queue link */
struct gendisk *bi_disk;
unsigned int bi_opf; /* bottom bits req flags,
* top bits REQ_OP. Use
* accessors.
*/
unsigned short bi_flags; /* status, etc and bvec pool number */
unsigned short bi_ioprio;
unsigned short bi_write_hint;
blk_status_t bi_status;
u8 bi_partno;
/* Number of segments in this BIO after
* physical address coalescing is performed.
*/
unsigned int bi_phys_segments;
/*
* To keep track of the max segment size, we account for the
* sizes of the first and last mergeable segments in this bio.
*/
unsigned int bi_seg_front_size;
unsigned int bi_seg_back_size;
struct bvec_iter bi_iter;
atomic_t __bi_remaining;
bio_end_io_t *bi_end_io;
void *bi_private;
#ifdef CONFIG_BLK_CGROUP
/*
* Optional ioc and css associated with this bio. Put on bio
* release. Read comment on top of bio_associate_current().
*/
struct io_context *bi_ioc;
struct cgroup_subsys_state *bi_css;
#ifdef CONFIG_BLK_DEV_THROTTLING_LOW
void *bi_cg_private;
struct blk_issue_stat bi_issue_stat;
#endif
#endif
union {
#if defined(CONFIG_BLK_DEV_INTEGRITY)
struct bio_integrity_payload *bi_integrity; /* data integrity */
#endif
};
unsigned short bi_vcnt; /* how many bio_vec's */
/*
* Everything starting with bi_max_vecs will be preserved by bio_reset()
*/
unsigned short bi_max_vecs; /* max bvl_vecs we can hold */
atomic_t __bi_cnt; /* pin count */
struct bio_vec *bi_io_vec; /* the actual vec list */
struct bio_set *bi_pool;
/*
* We can inline a number of vecs at the end of the bio, to avoid
* double allocations for a small number of bio_vecs. This member
* MUST obviously be kept at the very end of the bio.
*/
struct bio_vec bi_inline_vecs[0];
};里面根本没有bierror。 再翻翻以前版本的代码，在4.12的bio定义里发现了bierror： https://elixir.bootlin.com/linux/v4.12.14/source/include/linux/blk_types.h
应该是从4.13版本内核之后，bio的结构定义发生了变化。
struct bio {
struct bio *bi_next; /* request queue link */
struct block_device *bi_bdev;
int bi_error;
unsigned int bi_opf; /* bottom bits req flags,
* top bits REQ_OP. Use
* accessors.
*/
unsigned short bi_flags; /* status, etc and bvec pool number */
unsigned short bi_ioprio;解决
修改flashcache_subr.c，将 bi_error改成bi_status。
重新编译，ok。
更新：
flashcache_subr.c按如下修改，可以兼容以前版本的内核。
void
flashcache_bio_endio(struct bio *bio, int error,
struct cache_c *dmc, struct timeval *start_time)
{
if (unlikely(dmc->sysctl_io_latency_hist &&
start_time != NULL &&
start_time->tv_sec != 0))
flashcache_record_latency(dmc, start_time);
#if LINUX_VERSION_CODE < KERNEL_VERSION(2,6,24)
bio_endio(bio, bio->bi_size, error);
#elif LINUX_VERSION_CODE < KERNEL_VERSION(4,3,0)
bio_endio(bio, error);
#elif LINUX_VERSION_CODE < KERNEL_VERSION(4,13,0)
bio->bi_error = error;
bio_endio(bio);
#else
bio->bi_status = error;
bio_endio(bio);
#endif
}
