Alex Elsayed wrote: > Étienne BERSAC wrote: > >> Hi, >> >> I'm testing dm-cache for using a local SSD as a cache for a network >> volume. My goal is to test dm-cache behaviour when network is down. >> >> The device is shared through iSCSI. The first test is filling a huge >> file with dd from /dev/zero. So, this is sequential access. While dd is >> running, i remove the tun iface from the bridge on the host. iSCSI >> properly detects the network failure. >> >> I expected dm-cache to continue filling dirty blocks, even if the >> network device is blocking. But it doesn't. The status shows that dirty >> blocks count stays the same. dd is blocking. >> >> I'm wondering if there is some cache desactivation due to >> sequential_threshold. But increasing sequential_threshold did not help. >> Where does dm-cache blocks ? >> >> Is it possible to have dm-cache "bufferize" blocks for network >> failures ? >> >> Regards, > > Disclaimer: Not an expert, and not actually involved in writing bcache > > Well, since you didn't mention changing it, I suspect you are operating in > the default "writethrough" mode - this doesn't return to userspace until > the data is on the backing (iSCSI in your case) device. For bcache over a > network volume, this is the safe option, since the client machine dying > won't lose data. > > If you put bcache in writethrough mode, it may well do exactly as you > describe, but only if iSCSI itself recovers from the connection loss (and > possibly other caveats I'm not thinking of). If it doesn't, or if it does > but doesn't manage to maintain the exact state it was in on disconnection, > then you will lose data. My apologies, I didn't realize until I ready my own reply on the way back that this was on the topic of dm-cache rather than bcache. To properly respond, as I understand it dm-cache works by migrating chunks between the backing device and the faster cache. If your write would go to the backing device, it would block exactly as normal. (I hope I didn't get this egregiously wrong...) -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel