> (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.

{
    auto lock = getLock();
    if (shared->index_mark_cache)
        throw Exception("Index mark cache has been already created.", ErrorCodes::LOGICAL_ERROR);
    shared->index_mark_cache = std::make_shared<MarkCache>(cache_size_in_bytes);
}
MarkCachePtr Context::getIndexMarkCache() const
{
    auto lock = getLock();
    return shared->index_mark_cache;
}
void Context::dropIndexMarkCache() const
{
    auto lock = getLock();
    if (shared->index_mark_cache)
        shared->index_mark_cache->reset();
}
void Context::setMMappedFileCache(size_t cache_size_in_num_entries)
{
    auto lock = getLock();
    if (shared->mmap_cache)
        throw Exception("Mapped file cache has been already created.", ErrorCodes::LOGICAL_ERROR);
    shared->mmap_cache = std::make_shared<MMappedFileCache>(cache_size_in_num_entries);
}
MMappedFileCachePtr Context::getMMappedFileCache() const