= Problem
While examining a customer's system we noticed unreasonable space
usage from a few snapshots due to gang blocks. Under some further
analysis we discovered that the pool would create gang blocks because
all its disks had non-zero write error counts and they'd be skipped
for normal metaslab allocations due to the following if-clause in
`metaslab_alloc_dva()`:
```
/*
* Avoid writing single-copy data to a failing,
* non-redundant vdev, unless we've already tried all
* other vdevs.
*/
if ((vd->vdev_stat.vs_write_errors > 0 ||
vd->vdev_state < VDEV_STATE_HEALTHY) &&
d == 0 && !try_hard && vd->vdev_children == 0) {
metaslab_trace_add(zal, mg, NULL, psize, d,
TRACE_VDEV_ERROR, allocator);
goto next;
}
```
= Proposed Solution
Get rid of the predicate in the if-clause that checks the past
write errors of the selected vdev. We still try to allocate from
HEALTHY vdevs anyway by checking vdev_state so the past write
errors doesn't seem to help us (quite the opposite - it can cause
issues in long-lived pools like the one from our customer).
= Testing
I first created a pool with 3 vdevs:
```
$ zpool list -v volpool
NAME SIZE ALLOC FREE
volpool 22.5G 117M 22.4G
xvdb 7.99G 40.2M 7.46G
xvdc 7.99G 39.1M 7.46G
xvdd 7.99G 37.8M 7.46G
```
And used `zinject` like so with each one of them:
```
$ sudo zinject -d xvdb -e io -T write -f 0.1 volpool
```
And got the vdevs to the following state:
```
$ zpool status volpool
pool: volpool
state: ONLINE
status: One or more devices has experienced an unrecoverable error.
...<cropped>..
action: Determine if the device needs to be replaced, and clear the
...<cropped>..
config:
NAME STATE READ WRITE CKSUM
volpool ONLINE 0 0 0
xvdb ONLINE 0 1 0
xvdc ONLINE 0 1 0
xvdd ONLINE 0 4 0
```
I also double-checked their write error counters with sdb:
```
sdb> spa volpool | vdev | member vdev_stat.vs_write_errors
(uint64_t)0 # <---- this is the root vdev
(uint64_t)2
(uint64_t)1
(uint64_t)1
```
Then I checked that I the problem was reproduced in my VM as I the
gang count was growing in zdb as I was writting more data:
```
$ sudo zdb volpool | grep gang
ganged count: 1384
$ sudo zdb volpool | grep gang
ganged count: 1393
$ sudo zdb volpool | grep gang
ganged count: 1402
$ sudo zdb volpool | grep gang
ganged count: 1414
```
Then I updated my bits with this patch and the gang count stayed the
same.
Reviewed-by: Mark Maybee <mark.maybee@delphix.com>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Signed-off-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Closes#14003
Serapheim Dimitropoulos
37d5a3e04b