> (you don't have to strictly follow this form)

**Use case**

Sometimes, for really sophisticated scenarios it can be hard to control the amount of in-progress backups run externally. For the purpose of simpler control we can introduce server-level setting which disallows running concurrent backups of any type. 

Something like this in `config.xml` (enabled by default):
`<allow_concurrent_backups>true</allow_concurrent_backups>`

**Describe the solution you'd like**

It's important that backups can be executed in distributed way on multiple servers. Need to take it into account.

**Describe alternatives you've considered**
External check of `system.backups` table, but it can be complex.

{
    return getAccess()->getQuota();
}
std::optional<QuotaUsage> Context::getQuotaUsage() const
{
    return getAccess()->getQuotaUsage();
}
void Context::setCurrentProfile(const String & profile_name)
{
    auto lock = getLock();
    try
    {
        UUID profile_id = getAccessControl().getID<SettingsProfile>(profile_name);
        setCurrentProfile(profile_id);
    }
    catch (Exception & e)
    {
        e.addMessage(", while trying to set settings profile {}", profile_name);
        throw;
    }
}
void Context::setCurrentProfile(const UUID & profile_id)