Oleg> Recently I have decided to try out LVM cache feature on one of Oleg> our Dell NX3100 servers running CentOS 7.4.1708 with 110Tb disk Oleg> array (hardware RAID5 with H710 and H830 Dell adapters). Two Oleg> SSD disks each 256Gb are in hardware RAID1 using H710 adapter Oleg> with primary and extended partitions so I decided to make ~240Gb Oleg> LVM cache to see if system I/O may be improved. The server is Oleg> running Bareos storage daemon and beside sshd and Dell Oleg> OpenManage monitoring does not have any other services. Oleg> Unfortunately testing went not as I expected nonetheless at the Oleg> end system is up and running with no data corrupted.
Can you give more details about the system. Is this providing storage services (NFS) or is it just a backup server? How did you setup your LVM config and your cache config? Did you mirror the two SSDs using MD, then add the device into your VG and use that to setup the lvcache? I ask because I'm running lvcache at home on my main file/kvm server and I've never seen this problem. But! I suspect you're running a much older kernel, lvm config, etc. Please post the full details of your system if you can. Oleg> Initially I have tried the default writethrough mode and after Oleg> running dd reading test with 250Gb file got system unresponsive Oleg> for roughly 15min with cache allocation around 50%. Writing to Oleg> disks it seems speed up the system however marginally, so around Oleg> 10% on my tests and I did manage to pull more than 32Tb via Oleg> backup from different hosts and once system became unresponsive Oleg> to ssh and icmp requests however for a very short time. Can you run 'top' or 'vmstat -admt 10' on the console while you're running your tests to see what the system does? How does memory look on this system when you're NOT runnig lvcache? Do you have any swap space configured on the system? It might make sense to allocate 10-20gb of swap space. Oleg> I though it may be something with cache mode so switched to writeback Oleg> via lvconvert and run dd reading test again with 250Gb file however that Oleg> time everything went completely unexpected. System started to slow Oleg> responding for simple user interactions like list files and run top. And Oleg> then became completely unresponsive for about half an hours. Switching Oleg> to main console via iLO I saw a lot of OOM messages and kernel tried to Oleg> survive therefore randomly killed almost all processes. Eventually I Oleg> did manage to reboot and immediately uncached the array. Oleg> My question is about very strange behavior of LVM cache. Well, I may Oleg> expect no performance boost or even I/O degradation however I do not Oleg> expect run out of memory and than OOM kicks in. That server has only Oleg> 12Gb RAM however it does run only sshd, bareos SD daemon and OpenManange Oleg> java based monitoring system so no RAM problems were notices for last Oleg> few years running with our LVM cache. Oleg> Any ideas what may be wrong? I have second NX3200 server with similar Oleg> hardware setup and it would be switch to FreeBSD 11.1 with ZFS very time Oleg> soon however I may try to install CentOS 7.4 first and see if the Oleg> problem may be reproduced. Oleg> LVM2 installed is version lvm2-2.02.171-8.el7.x86_64. Oleg> Thank you! Oleg> Oleg Oleg> _______________________________________________ Oleg> linux-lvm mailing list Oleg> linux-lvm@redhat.com Oleg> https://www.redhat.com/mailman/listinfo/linux-lvm Oleg> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/ _______________________________________________ linux-lvm mailing list linux-lvm@redhat.com https://www.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
