Thanks for your reply.
I think there are 2 scenarios:
1. Peering for non-Primary OSD down
for example, PG [1, 3, 5] -> [1, 3, 6], osd 1 is always the primary OSD, and the data should be correct except data corruption, and we can read data directly from 1 before peered.
I think we can check "primay of last interval == primay of current interval", if equal, the primay haven't get changed, and we run into this scenario. Calcuation last interval and current
interval is much fast than peering, so we can first calc them then compare them.
2. Peering for primary OSD down
if primay osd down, "primay of last interval != primay of current interval", so we need waiting for peering.
-Xiaobing
发件人: ceph-devel-owner@xxxxxxxxxxxxxxx <ceph-devel-owner@xxxxxxxxxxxxxxx> 代表 Gregory Farnum <gfarnum@xxxxxxxxxx>
发送时间: 2018年3月19日 17:50
收件人: xsong682@xxxxxxxx
抄送: ceph-devel@xxxxxxxxxxxxxxx
主题: Re: Reading data before peered to improve performance
On Sat, Mar 17, 2018 at 8:06 AM, xsong682@xxxxxxxx <xsong682@xxxxxxxx> wrote:
> In current code, before PG peered, PG can’t do any operations including reading in following code
>
> void PrimaryLogPG::do_request( OpRequestRef& op, ThreadPool::TPHandle &handle)
> {
>
> if (!is_peered()) {
> // Delay unless PGBackend says it's ok
> if (pgbackend->can_handle_while_inactive(op)) {
> bool handled = pgbackend->handle_message(op);
> assert(handled);
> return;
> } else {
> waiting_for_peered.push_back(op);
> op->mark_delayed("waiting for peered");
> return;
> }
> }
>
> that will impact performance, I think the primary OSD control the data writing, if it haven’t got changed, the data on the primary OSD is always correct except silent data error, so the data can be read safely before peered.
> If it's ok, another question is how to know the OSD haven’t changed? Checking “primary of acting_set == primary of up_set” is enough or not? Should we check interval for the situation primary OSD change from “A-->B-->A”?
Knowing if the OSDs changed is what peering is for, and is why we
don’t allow reads on unpeered PGs. If we did allow you to read without
peering, you might read the data from an old and stale OSD that was
fenced off from the rest of the cluster and didn’t know it!
-Greg
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